The country's savings and loan associations last month experienced their biggest net withdrawal even, when savers took out $2.3 billion more than they deposited, according to figures released by the Federal Home Loan Bank Board.

Cognizant of its members' financial straits, the board last week authorized them to make mortgage loans that fluctuate with market rates. The return on these new mortgages is calculated over the long term to reduce the current drain on S&Ls saddled with existing loans yielding an average of 9 percent. A majority of S&L deposits now earn about 14 percent.

The previous record for withdrawals exceeding deposits was $1.5 billion, which occurred in July 1966 and April 1979. Because of inflation and the increase in total savings, however, the 1966 outflow was far greater proportionally.

The outflow in March erased gains in January and February and turned the first quarter to a negative $800 million, the first quarterly loss since the $2.9 billion outflow in the third quarter of 1974.

Most of the decrease came in $10,000 six-month money market certificate balances, which fell by $100 million. One year earlier, they had grown by $17.3 billion. S&Ls also experienced a decline of $800 million in jumbo certificates ($100,000 and over).

The thrift industry is convinced that the bulk of these withdrawals went to money market mutual funds, which were yielding several percentage points over banks and thrifts during March. Money-fund assets increased by $16.5 billion at the same time net S&L deposits sank by $2.3 billion.

The outflow announced by the Federal Home Loan Bank Board was considerably larger than the $1.7 billion predicted last week by the U.S. League of Savings Association. League economist Jim Christian said its figure was based on earlier federal estimates. Speaking of the advantage offered by money funds, he added, "For us to have lost only that much isn't half bad."

Christian added that despite the new mortgage type announced last week, lending could not be expected to pick up until deposits do. Loans closed by S&Ls totaled $4.7 billion in March, up 30 percent over February, but less than could be expected on a seasonally adjusted basis. The volume was the smallest since last June. On a seasonally adjusted basis, new commitment activity was the smallest since last spring.

This situation led economist Dale Riordan of the National Savings and Loan League to quip, "Some savings and loan CEOs have not made a mortgage loan in so long that they're getting out of practice on their loan-to-value ratios." Riordan calls the earnings outlook for S&Ls dim. He estimates that overall net worth declined $500 million to $600 million during the first quarter. (S&Ls still have a net worth of $30 billion.)

The associations are expected to lose 36 cents on every $100 of assets, according to Riordan, and the second quarter looks worse -- 50 cents. He predicts that if interest rates hold steady throughout the first half, the industry will be in the red to the tune of $1.25 billion to $1.5 billion. This is because of the large volume of lower-yielding certificates of deposits rolling over at higher rates.