Perpetual Federal Savings and Loan Association, the area's largest, lowered its mortgage interest rate yesterday from 15 to 13 percent, joining a growing national and local trend of sharply dropping mortgage rates.

At the same time, the U.S. Department of Housing and Urban Development announced yesterday a nationwide reduction in the maximum interest rate for FHA- and VA-insured mortgages from 13 percent to 11 1/2 percent for single-family homes -- the sharpest decline in FHA history.

Both actions were seen as efforts to help the depressed housing industry by attracting families who had been reluctant to buy homes at high interest rates. The Perpetual rate was 17 percent as little as one month ago.

Both the Perpetual and FHA-VA rate changes are effective today.

"We can't satisfy the entire Washington market, but we will do what we can," said Thomas Owen, president of Perpetual. "Our rate was too high at 15 percent, purposefully too high. I think the market can stand a 13 percent rate. Other lenders are moving in that direction, and this might put us, for a brief moment, slightly ahead of them."

Perpetual had raised its mortgage interest rate to 17 percent on March 5 in a deliberate attempt to discourage loan applications, and then lowered it to 15 percent about three weeks ago.But the 15 percent level still generated only "a limited amount" of loan demand, Owen said.

HUD officials estimated that dropping the VA-FHA ceiling will enable 3.5 million more families across the country to obtain mortgages. On a $60,000 mortgage, the difference between a 13 percent interest rate and an 11 1/2 percent interest rate is about $70 in monthly payments.

HUD Secretary Moon Landrieu also announced yesterday a temporary measure designed to help suffering builders reduce carrying costs on unsold homes.

Mortgage loan applications have picked up a bit as mortgage interest rates have declined in recent weeks.

Many potential buyers are not aware of the decline yet, lenders noted, and some who know of the reductions are waiting for rates to drop even further before buying. So, local officials expect applications to increase even more in coming weeks.

A little more than a month ago, interest rates peaked at most local lending institutions, hovering at between 16 1/2 percent and 17 percent according to Victor J. Peeke, who surveys 22 area savings and loans, banks, and mortgage companies each week.

Perpetual's 15 percent rate for conventional mortgages is not the lowest in the area. Peeke's survey showed several mortgage lending rates between 12 and 13 percent.

A small difference in interest rates can make a big difference in housing payments. A home buyer with a $75,000, 30-year mortgage would save nearly $200 a month in principal and interest payments by getting a loan at 12 3/4 percent interest instead of at 16 percent, for example.

As interest rates rose, residential real estate sales have slumped. Figures from board of realtors organizations in Northern Virginia and Prince George's County yesterday showed that sales last month through multiple listing services were less than half the total in April of last year.

But real estate agents say they now are encouraged by the brightening of the mortgage money picture.

"Business has picked up. . . . It's almost like somebody took the cork out of the bottle," said William M. Ellis, vice president of the Shannon & Luchs real estate company.

Ellis said that the real estate market during the first half of April was "dead, dead, dead. People just weren't buying. . . . It was the worst residential real estate market I've seen since I began my career in 1960."

Sales have increased since about April 20, he said. Every time the interest rate drops a fraction, more people can afford to buy. "Above 14 percent, a lot of people just couldn't buy," Ellis said.

Brain Logan of Long & Foster real estate said the office he manages on Capitol Hill is having a "very good" month.

"We're getting regular loans, but also assumptions and owner financing and VA and FHA loans," Logan said. "We're getting better turnout at open houses."

Declining interest rates nationally reflect an improvement in the money market picture. Home Savings and Loan Association in California, the nation's largest housing lender, lowered its mortgage interest rate a dramatic 4.75 points in one day last week -- from 17 1/2 to 12 3/4 -- to encourage home buying.

Home Savings has several hundred million dollars more than the amount of liquid assets, or cash investment, the law requires, and can rasie additional money through such means as loan sales, a spokesman said. The 12 3/4 percent level was set because most people seemed to drop out of the buying market when rates reached 13 percent, the spokesman said.

Locally, a spokesman for the Metropolitan Washington Savings and Loan League pointed out that even though local savings and loans still are experiencing heavy outflows of savings, more money is available now to loan -- at least temporarily.

Some lenders have accumulated funds from money paid back on existing mortgages, money that built up during recent months when few new loans were being made.

Most housing experts expect savings flow at savings and loan associations to improve in the coming months, said Kenneth Thygerson, chief economist for the U.S. League of Savings Associations.

"Mortgage interest rates are unfolding daily, literally daily," Thygerson said. "It's very hard to generalize. The market is in a state of economic flux."

Thygerson said rates appear to be between 12 percent and 16 1/2 percent nationally, with significant downward trends all over the country.

Most housing experts expect conventional interest rates to level off at about 12 percent sometime this year.

"It's hard to predict peaks and valleys," noted Owen, of Perpetual. "But I think we're getting close to the valleys."