The Navy Federal Credit Union began offering mortgage loans this week to its civilian and military members in Northern Virginia. It hopes to expand its lending activities to the District and Maryland before summer.

The largest federal credit union in the country, Navy FCU has assets of $750 million. Mortgage officer John Henderson said it is able to lend at a time when many other credit unions have ceased doing so because it already has $10 million in commitments on the secondary market.

In the first week, he added, many inquiries and three applications have been received. He anticipates that many of the applicants will be Navy personnel from out of state who are transferred to the Washington area.

Northern Virginia was selected for the initial mortgage-lending program because the credit union is located in Vienna, officials said. Members may apply for loans on primary residences containing not more than four units in Arlington, Fairfax, Loudon and Prince William counties and Alexandria. Prevailing market rates are offered: 10 3/4 percent, 30-year maturity plus 1/2 point to the buyer on conventional loans, and 9 1/2 percent plus five points to the seller and one point to the buyer for FHA and VA loans. Required down payments range from 10 percent down.

Federal credit union regulations do not permit loans on homes whose prices exceed 150 percent of the median sales price in the area. Henderson said the "area" means the county or some smaller jurisdiction.