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For years, investors cared little about risks in mortgage holdings. That is changing.

\ufffdI would not be surprised if between now and the end of the year at least 20 percent of BBB and BBB- bonds that are backed by subprime loans originated in 2006 will be downgraded,\ufffd Mr. Lawler said.

Still, the rating agencies have yet to downgrade large numbers of mortgage securities to reflect the market turmoil. Standard & Poor\ufffds has put 2 percent of the subprime loans it rates on watch for a downgrade, and Moody\ufffds said it has downgraded 1 percent to 2 percent of such mortgages that were issued in 2005 and 2006.

Fitch appears to be the most proactive, having downgraded 3.7 percent of subprime mortgages in the period.

The agencies say that they are confident that their ratings reflect reality in the mortgages they have analyzed and that they have required managers of mortgage pools with risky loans in them to increase the collateral. A spokesman for S.& P. said the firm made its ratings requirements more stringent for subprime issuers last summer and that they shored up the loans as a result.

Meeting with Wall Street analysts last week, Terry McGraw, chief executive of McGraw-Hill, the parent of S.& P., said the firm does not believe that loans made in 2006 will perform \ufffdas badly as some have suggested.\ufffd

Nevertheless, some investors wonder whether the rating agencies have the stomach to downgrade these securities because of the selling stampede that would follow. Many mortgage buyers cannot hold securities that are rated below investment grade \ufffd insurance companies are an example. So if the securities were downgraded, forced selling would ensue, further pressuring an already beleaguered market.

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On March 2, reacting to the distress in the mortgage market, a throng of regulators, including the Federal Reserve Board, asked lenders to tighten their policies on lending to those with questionable credit. Late last week, WMC Mortgage, General Electric\ufffds subprime mortgage arm, said it would no longer make loans with no down payments.

Meanwhile, investors wait to see whether the spring home selling season will shore up the mortgage market. If home prices do not appreciate or if they fall, defaults will rise, and pension funds and others that embraced the mortgage securities market will have to record losses. And they will likely retreat from the market, analysts said, affecting consumers and the overall economy.

A paper published last month by Mr. Rosner and Joseph R. Mason, an associate professor of finance at Drexel University\ufffds LeBow College of Business, assessed the potential problems associated with disruptions in the mortgage securities market. They wrote: \ufffdDecreased funding for residential mortgage-backed securities could set off a downward spiral in credit availability that can deprive individuals of home ownership and substantially hurt the U.S. economy.\ufffd