WASHINGTON - Rates on 30-year-fixed mortgages dropped this week to their lowest levels in at least 37 years, as the Federal Reserve pledged to pour money into the mortgage market in an effort to spur the moribund U.S. housing market.

Freddie Mac, the mortgage company, reported yesterday that average rates on 30-year fixed-rate mortgages dropped to 5.19 percent, down from the year's previous low of 5.47 percent, set last week.

The rate is the lowest since Freddie Mac's weekly mortgage rate survey began in April 1971.

Mortgage rates started falling after the Federal Reserve launched a sweeping new effort in late November to aid the housing market by buying up to $600 billion of mortgage-related securities and other debt issued by Fannie Mae, Freddie Mac and the Federal Home Loan Banks.

A daily survey found that the national average rate fell even lower Wednesday. Rates on 30-year, fixed mortgages was 5.06 percent, according to financial publisher HSH Associates, the lowest since the 1960s and down from 5.3 percent Tuesday.

It was the best news in months for anyone looking to lock in a 30-year, fixed-rate mortgage. But it was not expected to be a cure-all, and borrowers already in danger of foreclosure probably won't be able to take advantage because only borrowers with stellar credit can qualify.

"It's a call to action for homeowners looking to get out of adjustable-rate mortgages," said Greg McBride, senior financial analyst at Bankrate.com. "Unfortunately, it's not an equal-opportunity party."

Faced with a dramatic surge in defaults, both Freddie and its sibling company, Fannie Mae, are stepping up efforts to prevent foreclosures.

The average rate on a 15-year fixed-rate mortgage dropped to 4.92 percent from 5.2 percent last week, Freddie Mac said.

Rates on five-year, adjustable-rate mortgages fell to 5.6 percent, compared with 5.82 percent last week. Rates on one-year, adjustable-rate mortgages dropped to 4.94 percent, from 5.09 percent last week. *