Perpetual Federal Savings and Loan Association, the Washington area's largest thrift institution, reported yesterday the first net loss for any of its 98 years in business.

Reflecting profound pressures now affecting the nation's S&Ls as they struggle with record interest rates and evaporating profit margins, Perpetual suffered a loss of $6.3 million for the year ended Oct. 31.

The unprecedented loss for Perpetual resulted from a decision to sell off mortgages with low interest rates -- thus incurring a one-time, non-operating loss but also making more mortgage money available to Washington area consumers at this time.

In August, Perpetual sold $130 million of its lower-yielding residential mortgages to Salomon Brothers, the New York investment firm. This transaction is one of the largest discounted secondary market transactions ever made by a federally chartered S&L.

The loss on selling a portion of the mortgage portfolio was $23.3 million, but this was offset partially by $10.2 million of federal income tax benefits.

Not counting this one-time transaction, Perpetual's operating income before taxes was $14 million in the recent year, compared with $16.5 million in the previous fiscal period. Revenues rose to $90.2 million from $78.5 million.

In a statement released yesterday, Perpetual said the decline in pre-tax profits reflected increased interest expense on certificate and regular savings accounts as well as interest paid on borrowed funds.

The $6.3 million net loss was charged against reserves, which stood at $90 million on Oct. 31.

"The loan sale (to Salomon Brothers) was another means of generating funds for mortgages," said Perpetual President Thomas J. Owen. The reduction of reserves "is more than offset by the present and future positive effects of the sale," he added.

Perpetual's reserve-to-savings ratio after the one-time charge still exceeds 11 percent, which Owen said may still rank the traditionally conservative Washington S&L in first place among the nation's 100 largest savings associations.

"With our strong reserve position Perpetual is one of only a few large savings and loans in the country that could easily accommodate this type of transaction," he added.

Perpetual assets on Oct. 31 were $1.035 million, down slightly from $1.039 billion a year ago. Savings deposits declined to $804 million from $807 million.

Reflecting the sale of some mortgages, Perpetual's portfolio of loans stood at $882 million on Oct. 31, compared with $941 million a year ago.

Perpetual spokesman Albert Auer said yesterday the S&L is back in the D.C. residential mortgage market after congressional action to approve a 15 percent interest rate ceiling in the city. Perpetual also remains active in the suburbs but record interest rates has led to a lower loan demand.

Auer said rates vary by jurisdiction, but he said an 80 percent loan (20 percent down payment) was 14 1/8 percent in early November with a maximum loan of $150,000.