Sumner Redstone said he may break up Viacom, the broadcast and advertising company he spent 18 years building, because investors aren’t placing a high enough value on the businesses.

Co-president Leslie Moonves would lead one company containing the CBS television network and Infinity radio units, Mr. Redstone said yesterday in an interview. Co-president Tom Freston would take the “high-growth” MTV cable networks. Mr. Redstone said he will be chairman of both businesses.

Shares of Viacom, the no. 3 American broadcasting, advertising, and publishing company, dropped four out of the past five years, dragged down by the tumbling value of its radio unit. The cable-TV assets would be worth more if they were free from slower-growing units, Merrill Lynch analyst Jessica Reif Cohen wrote yesterday. The split would also allow Mr. Redstone, 81, to avoid picking Mr. Moonves, 55, or Mr. Freston, 59, as a successor.

“If the stock continues to languish below what we consider fair value, we believe Viacom should consider breaking up the assets to unlock the underlying value,” Ms. Reif Cohen wrote before Viacom’s announcement.

Class B shares of New York-based Viacom rose $2.13, or 6.3%, to $36 in New York Stock Exchange composite trading, the biggest gain in two years, after the Wall Street Journal reported the plans. The shares have fallen 3% in the past year, compared with a 7% gain for the Standard & Poor’s 500 Index.

Viacom wrote down the value of radio and billboard assets by $18 billion in the fourth quarter.

Viacom’s businesses have “inherently different growth characteristics,” Mr. Redstone said in a statement. The “distinction is likely to continue to limit Viacom’s ability to receive full value for its assets and its prospects in the investment community,” Mr. Redstone said.

Mr. Freston would take control of a company with less revenue than Mr. Moonves though higher growth, Mr. Redstone said. A decision will be made next quarter, said Mr. Redstone, who turns 82 in May. Mr. Redstone last month said his daughter Shari Redstone, 50, is likely to control Viacom when she inherits his stake in the company.

The division proposed by Mr. Redstone would put Mr. Freston in charge of operations that accounted for 29% of the company’s sales.

Viacom’s cable networks include MTV, Nickelodeon, and VH1.

Mr. Freston would also take the Paramount movie studio, the seventh ranked last year in box-office revenue, Ms. Reif Cohen said.

Mr. Freston’s company would then have currency for acquisitions, Mr. Redstone said.

Mr. Moonves would have businesses that accounted for 55% of Viacom’s sales. CBS is the top-rated television network among audiences this season and Infinity radio is the no. 2 American radio company. That business would also include the billboard business.

Mr. Moonves’s business would be more likely to buy back shares and pay dividends, Mr. Redstone said.

Viacom, no. 3 behind Time Warner and no. 2 Walt Disney in terms of revenue, also owns the UPN broadcast network and the Simon & Schuster publishing house.

The company could revive its stock price by reporting better-than-expected first-quarter results and selling some of its radio stations and theme parks, including Paramount’s Kings Island in Cincinnati and Paramount Canada’s Wonderland in Toronto, Ms. Reif Cohen wrote.

Viacom plans to put its Infinity Broadcasting unit on a “growth path” by focusing on stations in the top 25 markets and investing in programming and marketing, Mr. Moonves said on a February 24 conference call to discuss earnings. Clear Channel Communications is the largest American radio broadcaster.

Sales and swaps of Viacom’s radio stations could begin in the second quarter, Ms. Reif Cohen wrote. The sale of about 70 stations could bring Viacom as much as $3.3 billion in pre-tax proceeds, she said.