A pleasant spring fever of new investment has Northern Virginia's financial giants in its grip, and nobody in the region's banking and housing industries wants a cure.

Bankers, savings and loan officials and mortgage companies are crediting a steady six-month decline in mortgage interest rates for rejuvenating the area housing industry and giving local real estate agents--and first-time home buyers--one of their best seasons ever.

The drop in interest on 30-year, fixed-rate mortgages from more than 16 percent a year ago to as low as 11 1/2 percent at some banks "is blowing up a lot of business this spring," said John L. Adams, principal broker at Avis, Adams and Associates Realtors in Vienna.

"It's a great break for the Realtors and the people who are looking for a house," said Adams, who said his business is up 40 to 50 percent since last year. "The mortgage rates have qualified a lot of people to buy homes who have not been able to qualify previously."

Adams isn't the only Northern Virginia real estate agent who is smiling these days at the plummeting mortage rates. New home sales are mushrooming across the region, according to the Northern Virginia Board of Realtors.

In March, for instance, when interest rates hovered between 12.8 and 12.9 percent, 1,800 single-family homes, town houses and condominiums were sold in Northern Virginia, for a 49 percent increase over the 1,211 sold in March 1982, the board said.

Then, in April, as rates dropped to 12.7 percent, home sales climbed again to 1,846--a 67 percent increase over the 1,105 sold in the same month last year, the board said.

Adams opened his firm in 1981 at what he called "the worst possible time for this market," and the good news could not be more welcome. He said his office in Vienna's largest shopping center is getting more and younger walk-in customers.

Two years ago this month, the rate on the Northern Virginia mortgage roller coaster was 16.35 percent, said Vincent J. Peeke, who publishes Interest Data Reports, a survey of the 80 leading lenders in the area.

By May 1982, the regional average had climbed to 16.55 percent, peaking two months later at nearly 17 percent. By the last half of August, rates had dropped by a point or more--and the decline remained fairly steady from there, said Peeke, who has been surveying lenders since 1978.

"It's been a long time since rates were this favorable," Peeke said. "For some institutions, this is the best season since April of '80."

Since mid-Decmber, when Virginia rates averaged 13.42, mortgage interest has dropped with virtually no interruption: 13.24 percent on Jan. 6; 13.32 on Feb. 3; 12.84 on March 3; 12.85 on April 7. The biggest weekly change occurred between April 29 and May 5 when the average rate dropped from 12.70 to 12.53, Peeke said. Meanwhile, interest on a FHA-VA mortgage, one of the area's most popular home financing methods, had declined to 11.5 percent.

Market analysts say there are at least two reasons for the drop in interest rates and the resulting surge in home buying. For the moment, at least, Northern Virginia is reaping some of the benefits from the economic recovery that started in January, economists say. A key element in that recovery, they add, has been the Federal Reserve Board's tight monetary policy to control inflation.

"The short-term effect of that restricted-money policy was to raise interest rates," said Warren Matthews, senior economist for the Mortgage Bankers Association in the District, "but the economy has been recovering. Lower mortgage rates are part of that cycle."

The cycle has spurred first-time home purchases and "move-up" buying by homeowners who can now afford larger houses, Matthews said. At many local banks, the number of loans and refinancings have begun to perk up, he said.

At the McLean-based Dominion Federal Savings and Loan Association, the number of residential loans the bank sold doubled in six months, said Tim Bolger, vice president for secondary marketing.

Recently, Dominion Federal has offered 12.25 to 12.5 percent interest on its conventional loans, Bolger said.

Even if mortgage rates drop to 11 percent--and many analysts say that could happen this summer--the regional economy's outlook is not completely rosy, according to Bolger and other analysts.

"We're a year and half, two years out of a depression," Bolger said, "and there's a lot of adverse investment psychology out there. Even with rates as low as they are, they may have to drop even further for that reluctance to disappear and even then it may not happen."

Still, if mortgage rates drop no lower than 11 percent, Virginia banks, realty firms and home buyers will have enjoyed a boom season, analysts said.

"We're fairly optimistic about the rates," Bolger said. "But it's sort of like being on a seesaw: There are ups and downs."