Recent sharp decreases in mortgage interest rates, combined with a pent-up demand for ownership housing since last October, have stimulated sales of both new and existing homes in the Washington region during the past two weeks.

While the residential housing market has not regained the hot buying pace of 1977-78 and early 1979, there are indications that the fall of conventional mortgage rates below 13 percent and the FHA-VA rate now at 11 1/2 percent have brought back prospective buyers who had been sitting on their checkbooks. Most of them simply were unable or unwilling to accept mortgage rates over 14 percent.

"We saw a definite turnaround in mid-May, just before the holiday weekend," said Tom Shafran, president of Better Homes Realty. His view was supported by several other brokers and builders questioned late last week. All attributed the turnaround to leave rates that released pent-up demand.

Leonard Whitecar, a Routh Robbins executive, said the "shocking drop in rates" brought the market out of the doldrums.

Kenneth J. Luchs, president of the Washington Board of Realtors, agreed with Whitecar that some pending transactions have been revived throughout this area. But Luchs warned that a hotter selling pace could turn this buyers' market into a sellers' market -- with the resultant higher prices -- if mortgage money remains available. He cautioned that rates could rise again just as rapidly as they went down.

Builders of new homes also confirmed the surge in sales. Arthur Titus, regional vice president for Ryan Homes, said that 90 sales contracts and deposits were taken in May for new homes in Newington Forest, a large northern Virginia community where prices begin at $64,000 for single houses and at $49,000 for town houses.

Many of the recent sales are made with FHA or VA financing that has been used increasingly for both new home subdivisions and resales. Mortgages insured by the Federal Housing Administration or guaranteed by the Veterans Administration led the rate downturn, going from a record 14 percent to an 11.5 percent ceiling in less than a month. That is because those rates were attractive to investors.

However, there are still reports that the residential rebound is not unanimous. For instance, Don Edwards (DEAR Inc.), a broker whose offices are in the District and suburban Maryland, said that sales increased only a "trickle" in recent weeks. He suggested that some buyers are waiting for borrowing rates to drop more.

Walter Hudson, general manager of Mike Casey Realtors in Prince George's and Anne Arundel counties, said sales are still slow despite attractive listings. He blamed a public perception of a "recession psychology."

Larry Silverman, head of a large realty firm that handles both new homes and resales, described recent business as "suprisingly good and steady." He said the condominium market is strong but that new homes sales are "spotty" except among the lower priced homes for first-time buyers.

Like others, Silverman contended that the market prices and financing are now affordable. Walter Johnson, a partner in District-Maryland Realty, said that homes priced right are selling and that a rebound was noted in May after a slow April. Almost all builders and brokers agreed that high rates killed the area market in April.

Speaking about high-priced home sales in Potomac, agent Maxine Schwartzman of Snider Bros. volunteered that she had sold three new houses, all priced above $279,000, in the past 10 days. "It's not financing or bad times in this price range," she contended. "Wealthy buyers are still willing and able to pay for quality and style."

But more attractive financing does stimulate lesser priced houses. Savings and loan executives Connie Harrell and Lamond Jones pointed out that area S&L's are making new mortgages on a limited basis and that old, low-interest mortgages held by sellers are now being converted to new loans for buyers at a "compromise" rate that is higher than the original but lower than the market rate, now around 13 percent.

S&Ls also are not holding buyers who agreed to higher rate loans during March and April for future closings. Now that money is available at lower rates, the loan can be sold off in the secondary market at lesser rates.

But funds still are a problem with S&Ls that are waiting for passbook savings to rebound. They are also hurting because long-term accounts and certificates are now paying rates now nearly 5 percentage points below the peak hit earlier this year.

Meanwhile, the National Association of Realtors reported that a survey of 22 metropolitan areas, including Washington, indicates a modest upturn in buying more looking traffic and currently stabilized prices.

Area Realtors Wesley Foster and Howard Rooks agreed that sales increased substantially in May. Rooks added: "Some prospective buyers may wait too long for rates and prices to bottom. There's little inventory of unsold new houses and any kind of a new buying spree could result in the usual pattern of price increases again."