WASHINGTON — Rates on 30-year-fixed mortgages fell 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.

Meanwhile, rates this week fell on 15-year fixed-rate mortgages to an average of 4.92 percent, down 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.