The next threat to American finance may involve money market funds, a heretofore stable financial strategy that could now be imperiled.

The trouble started on Tuesday, when the country’s oldest money market fund, the Reserve Primary Fund, announced it was strained to the point where investors could not recoup all the cash that they had put in. The fund had issued $785 million in short-term loans to Lehman Brothers Holdings, which had gone bankrupt, meaning the money market was unable to recoup the money. It was only the second time in the history of money markets that investors of a fund were exposed to a loss; money markets are traditionally considered as safe as having cash in the bank.

As concern over Reserve Primary Fund’s failure reverberated through Wall Street yesterday, money managers rushed to stop investors from fleeing, working to reassure them that their balance sheets are far more robust than Reserve Primary Fund’s. To prevent running into a similar problem, these funds appeared to sharply restricted their short-term loans to companies such as UBS, Sears, and GMAC.

“The fear that a lot of people have is, what if people panic and start withdrawing money en masse from money market funds?” the managing director of the investment research company Morningstar, Don Phillips, said. “The whole money market business, overnight, became dramatically more conservative, which will raise the cost of borrowing for lots of companies.”

Short-term debt costs spiked yesterday, according to an analysis by Bloomberg News, with GMAC paying more for short-term loans than it has all year.

The rising cost of short-term loans could exacerbate the global credit crisis that has already seen the collapse of three venerated financial institutions so far this week.

It was unclear as of yesterday evening whether the money market funds had managed to avoid investor panic. Industry insiders said the feared exodus did not seem to occur, and no other funds announced a devaluation like the one Reserve Primary Fund experienced.

“It’s unlikely that any additional funds will ‘break the buck’ in the very near future,” money market tracker Peter Crane said, referring to the industry term for a devaluation. “Whether investors will blink remains to be seen.”

Investors withdrew only $7 billion from money funds on Tuesday, according to Mr. Crane’s survey data. That is a relatively meager figure given that the industry manages well over $3 trillion. Data for yesterday was not yet available, Mr. Crane said.

Some of the country’s top mutual funds, which control much of the money fund market, released statements assuring investors that they had limited exposure to Lehman Brothers and had taken steps to cover any foreseeable losses.

“I don’t think it poses any major danger to the money market fund industry or to the financial system,” a retired president of the trade group Investment Company Institute, Matthew Fink, said of Reserve Primary Fund’s devaluation. “In hindsight, someone was going to do it.”

The value of Reserve Primary Fund’s share price fell to 97 cents on Tuesday, dipping below the $1 value that all money market funds are intended to maintain. Money markets provide their shareholders only with dividends, not with increases in value that are typical of many other investments.

Reserve Primary Fund’s management was unwilling or unable to cover the loss for the fund, which had assets of $65 billion as of this summer. On occasion, some other money market funds have avoided similar losses because of contributions from their managers. Reserve Primary Fund declined to comment.