State National Bank, which has its headquarters in Rockville, has become the first area bank to offer so-called "loophole" certificates. A borrowing plan enables savers with less than $10,000 to receive near-market rates on six-month deposits.

For example, if a depositor has $4,000, the bank lends an additional $6,000 needed to reach $10,000, the minimum amount required to purchase a money market certificate pegged to Treasury bill rates. Although the customer pays interest on the $6,000 loan (one percent over interest paid on deposits), the substantially higher return offered on $10,000 money market certificates enables him or her to earn a much higher effective yield than the customer could earn on the $4,000 alone.

The maximum amount a bank is permitted to pay on a $4,000 six-month deposit is 5.5 percent. The rate this week on a $10,000 six-month certificate is 11.716 percent, yielding 12.266 percent if the depositor puts up the entire sum. If he or she borrows the $6,000, his yield is 11.591 percent, or more than twice as much as he or she ordinarily would receive on a $4,000 deposit.

No withdrawal is permitted before maturity except in the case of death or a declaration of incompetency. Otherwise the depositor, subject to penalties on interest earned on the deposit, could wind up owing the bank money on the interest due on the loan.

The yields on loophole certificates are competitive with those paid by money market funds, although the initial deposit of $4,000 is larger than required by most money market funds. Also, the funds offer daily liquidity; on the other hand, the customer's money isn't insured against loss, whereas the principal and rate are guaranteed on a bank certificate.

The new instrument is known as a loophole certificate because nothing in federal regulations specifically permitted or disallowed it when the idea originated in the summer of 1978. It was not until more than a year later that federal regulators declared formally that they had no legal objections to the concept.

In recent weeks, banks in New Hampshire, New York, Pennsylvania and Nebraska have announced they would offer loophole certificates. The Casco Bank and Trust Co. of Portland, Me., which initiated loopholes, said it has had 100 calls this month from interested banks.

Savings and loan associations have been prohibited from offering them.The Federal Home Loan Bank Board recently told a Seattle S&L that "it was not within the spirit of the law or the regulations to subvert that with such devices." The loophole, of course, allows small savers to take advantage of rates normally paid only on larger deposits.