Whatever happened to Maurizio Cattelan? The market for the New York-based Italian conceptual artist seemed unstoppable in the fall of 2004, when his auction record was broken three times, resting at $3 million. But Mr. Cattelan’s stock plummeted abruptly last spring, when two works estimated to fetch over $1 million each failed to sell at Christie’s. This season’s contemporary art evening sales feature no Cattelans at all.

The rise and fall of Mr. Cattelan at auction illustrates that trendiness can afflict even the most critically acclaimed artist. Auction houses take risks each season betting on whom the buying public still craves and whose prices they can push even higher.

Christie’s launches this week’s auctions of Postwar and Contemporary art on Tuesday evening with a passel of works by Donald Judd. The Donald Judd Foundation is unloading 26 works at the evening sale, which are estimated to sell for up to $21.7 million; the flood of Judds could either ignite latent passions for his work or dampen enthusiasm.

Presale estimates for hot, rising, or rediscovered artists often test the lengths to which buyers will go. At this week’s auctions, even the presale low estimates for works by David Hockney, Damien Hirst, Andreas Gursky, Roy Lichtenstein, and Robert Ryman would be new records for these artists. Not far behind are the low estimates for Mike Kelley, Christopher Wool, and Wilhelm Sasnal, which hover near the artists’ current auction records.

One of the more arresting works in the Christie’s evening sale is Mr. Hirst’s “Away From the Flock, Divided” (1995). The sheep, sliced in two and preserved in aqua-colored formaldehyde, is estimated to sell for between $3 million and $3.5 million, far above Mr. Hirst’s auction record of $1.3 million set last season at Sotheby’s for a painting made of butterfly wings. But “Away From the Flock” is the first full preserved-animal installation by Mr. Hirst to come up at auction, and others have reportedly sold privately for as much as $8 million.

The plangent brassiness of Mr. Hirst’s lamb is matched by Jeff Koons’s bronze “Aqualung” (1985), which is estimated at $2.5 million to $3.5 million, in line with past sales for works from this series. His bronze “Lifeboat” (1985) sold for $3.4 million last season at Sotheby’s. An edition of “Aqualung,” of which three were made,sold for $1.8 million at Phillips, de Pury in May 2002.

If works by Mr. Koons continue to do well in these sales, he could further cement his place as the true heir of Warhol – fey, flip, addicted to pop culture, and eternally saleable. The cover lot for Sotheby’s Wednesday evening sale is Mr. Koons’s “New Hoover Convertibles, Green, Red, Brown, New Hoover Deluxe Polishers Yellow, Brown Doubledecker” (1987) – the title of which precisely describes the work – estimated between $2.5 million and $3.5 million. A “New Hoover”work with just two vacuums sold for $2.6 million at Christie’s in November 2004.

The Christie’s sale, estimated to earn between $113 million and $160.7 million, will also test the depth of passion for Mr. Hockney’s sunny, bleached realism. Mr. Hockney’s wall-sized “A Neat Lawn” (1967), a view of sprinklers in front of a 1960s-era Californian apartment complex, is estimated to make between $3.5 million and $4.5 million.

“It’s exactly at the moment he goes to Los Angeles, and it’s still fresh for him,” a Christie’s co-head of Postwar and Contemporary art,Amy Cappellazzo, said of “A Neat Lawn.” It was painted the same year as “A Bigger Splash,” an iconic Hockney owned by the Tate Museum and cited by Ms. Cappellazzo as a companion picture. Mr. Hockney’s auction record of $3.2 million was set last June in London.

Christie’s is also betting that appetites are whetted for those mid-century European artists who straddled the shift from painting to conceptualism. “[Yves] Klein, [Piero] Manzoni, and [Lucio] Fontana are seen as undervalued,” the other co-head of Postwar and Contemporary Art at Christie’s, Brett Gorvy, said. He added that there has been a strong American response of late to artists traditionally coveted by European collectors.

The estimate, between $3.5 million and $4.5 million, for Klein’s “ANT 127” (1960) would be a record for a work from the “Anthropometrie” series, but Mr. Gorvy said works have sold for more privately. Christie’s is also hoping to bring in between $4.5 million and $6.5 million for one of Klein’s delicate bluesponge “RE” paintings, for which the record is $5.3 million.

Leading Sotheby’s evening sale on Wednesday night is Roy Lichtenstein’s “Sinking Sun” (1964), which the auction house expects to draw upwards of $20 million. Lichtenstein’s “In the Car” brought $16.3 million last season at Christie’s, a huge breakthrough for the artist’s price at auction.

Sotheby’s worldwide head of contemporary art, Tobias Meyer, believes “Sinking Sun” is just as important. “It is the last frame after a couple kisses. It looks west toward the future, and the romantic notion of ‘Go West young man,’ ” Mr. Meyer said at a presale press conference. The entire sale is estimated to earn between $93.7 million and $126.2 million.

Sotheby’s is also bullish on photographer Andreas Gursky, hoping for a big record for his “99 Cent” (1999), a kaleidoscopic image of discount store goods. The estimate, between $1 million and $1.5 million, is well above Mr. Gursky’s auction record of $632,000. But a Sotheby’s postwar and contemporary art specialist, Alexander Rotter, says this particular image is the last available of an edition of six and is among Mr. Gursky’s most iconic, along with “Paris, Montparnasse” and “Rhine.”

Then there are the postwar artists who entered the art-history books decades ago but are seeing market spikes now: David Smith, thanks to the Guggenheim retrospective and the $23.8 million sale of “Cubi XXVIII” (1965) at Sotheby’s last fall; Francis Bacon, thanks to the art world’s renewed attention to figuration; and Morris Louis, thanks to a dearth of top-quality Abstract Expressionist pieces.

Both Christie’s and Sotheby’s are partaking of the Willem de Kooning juggernaut. The renewed respect for a painter who long played second fiddle to Jackson Pollock has been spurred in part by the publication of a Pulitzerwinning biography by Mark Stevens and Annalyn Swan, as well as last fall’s sale of pictures owned by de Kooning’s lawyer, Lee Eastman, at Christie’s. At that time, an airy, blue-and-white-dominated untitled work from 1977 sold for $10.7 to dealer Robert Mnuchin, who was bidding on behalf of his client Steven Cohen.

“Untitled,” which set a record for a de Kooning work from the 1970s, is now the centerpiece of a non-selling exhibition at Mr. Mnuchin’s gallery, L&M Arts, featuring privately owned works from that period. Sotheby’s is selling another 1970s de Kooning, “Untitled XVI” (1975), a more frenetic and denser celebration of color, estimated between $6.5 million and $8.5 million. At Christie’s, an earlier but similarly big-stroked fleshy abstraction, “Untitled” (1961), is expected to make between $8 million and $12 million.

“Sure, there’s been a change in appreciation and recognition of the relative beauty and importance of this period over his career,” said Mr. Mnuchin, whose inaugural 1993 exhibition was also of mid-1970s de Koonings. “We could debate forever whether Pollock at his height or de Kooning was more important. But de Kooning had four or five periods, each noticeably different from the rest, which is so hard to do.”

Three works by Robert Ryman, who for five decades has been quietly exploring the varied ways in which white paint can be expressive, are appearing in the evening sales: two at Sotheby’s and one at Phillips. Sotheby’s estimates Mr. Ryman’s “Untitled” (1962) will make at least $4 million, double his existing auction record.The artist’s dealer, Arne Glimcher, the chairman of PaceWildenstein, said via email that Rymans rarely come up at auction, because “they are usually collected by true collectors and not by speculators.”

Christie’s Postwar and Contemporary Art evening sale is held Tuesday, May 9, at 6:30 p.m. at 20 Rockefeller Center. Viewing is held today from 10 a.m. to 5 p.m. and tomorrow from 10 a.m. to noon.

Sotheby’s Contemporary Art evening sale is held Wednesday, May 10, at 7 p.m. at 1334 York Avenue. Viewing is held today and tomorrow from 10 a.m. to 5 p.m. and Wednesday from 10 a.m. to noon.

Phillips, De Pury and Company’s Contemporary Art evening sale is held Thursday, May 11, at 7 p.m. at 450 W. 15th Street.Viewing is today through Wednesday, 10 a.m. to 5 p.m., and Thursday, 10 a.m. to 1 p.m.