Sen. Laurence Levitan, chairman of the senate's budget and tax committee, said today he will introduce legislation that would limit the interest rate The American Express Co. can charge customers using its new extended-payment plan.

The company, which has more than 200,000 cardholders in Maryland, last month introduced the extended-credit plan, on expenditures involving travel, charging an annual interest rate of 21 percent to cardholders in Maryland, the District and 15 other states.

In the other 34 states, cardholders only pay 18 percent annually.

Levitan's bill would make it illegal for the company to charge a higher percentage rate here than it does in the majority of states.

American Express said it began the extended-credit service in response to other credit card companies that charge an annual fee lower than its $35 charge and in some cases do not require full payment of bills upon receipt, as American Express has always done.

James Doyle, who is the lobbyist for American Express here, said today the higher rate in Maryland results from the company's use of a bank as a middleman between itself and its cardholders.

"The company charges 18 percent in states where it can deal directly with the cardholder," Doyle said. "In states where it has to deal through a bank it charges whatever rate it can get from the bank. The best rate it was able to get with a bank was 21 percent."

F.D. Wilkinson Jr., an American Express vice president, said the higher rate is the result of "ambiguities" in the state's credit laws.

"Rather than not offer the service in those states, we approached a bank Security Pacific National of Los Angeles about being the creditor on these loans. They agreed and they set the rate," Wilkinson said.

Asked what American Express would do if Levitan's bill were passed, Wilkinson said, "Until we actually see the legislation and what it would do, I would rather not comment."

"There must be a way we can do something so people don't have to pay more than the 18 percent," Levitan said. "If this bill doesn't provide the entire answer, it's at least a start. I would like to know more about why a bank is involved and why it is setting a higher interest rate than the one the company is setting.

"At the very least, I think we need to have some public hearings on the matter so we can figure out what we need to do to get this straight. I just can't see any justification for people . . . paying that extra three percent."

Levitan said he got the idea for legislation after receiving a form on the new extended-credit service and noticing the difference in rates.