The Carter administration today took an important first step designed to prevent U.S. multinational banks from becoming overextended in their lending operations in foreign countries.

John G. Heimann, comptroller of the currency, proposed a formal rule that will bar a bank from lending more than 10 per cent of its capital to a foreign government. The comptroller's office regulates the activitities of the 4,500 national banks.

A foreign government and any of its agencies or instrumentalities would count as one borrower, unless it can be shown that a separate entity has its own resources to pay off a loan.

The move comes in the context of increased nervousness over the exposure of American banks because of their loans to foreign governments, which have zoomed since the 500 per cent increase in oil prices since 1973. American banks reportedly have more than $50 billion in loans outstanding to less-developed countries.

Essentially, the 10 per cent limit has been in force for national banks under an 1863 law, last amended in 1864. But there has never been a formal regulation, for have banks been required to show with any great precision that individual borrowing entities have actually had their own means of repayment.

Thus, according to the comptroller's office, some banks may actually have exceeded the 10 per cent limit. Under the proposed new regulation they would be required to justify the gross volume of their loans.

Heimann said at a press conference that the regulation "was not brought up by any crisis." But he and other officials stressed that with an enormous and growing volume of loans by American banks to less-developed countries, it was necessary to "anticipate" a possible overextension by some American banks.

Not affected are state banks, under the control of the Federal Deposit insurance Corp. Several large state banks in New York and California operate on a 25 per cent aggregate limit rule. Of more than 14,000 national and state banks in the United States, only 141 - the big mulinationals - are involved in foreign lending.

Foreign countries such as Mexico have been concerned that the United States would take steps to curb lending by American banks. Coincidentially, some American banks that have sought to limit new commitments have, according to Heimann, "blamed the regulators."

The new regulation, he said, "puts everybody on notice that there are limits." But he emphasized that it is a bank's responsibility to determine the degree to which it chooses to be exposed in any one country.

Officials cited a speech last May to a bankers' group by Deputy Comptroller Robert R. Bench in which he reminded bank directors of their responsbility to manage a bank's affairs. "Directors understand that, unlike countries, banks can disappear," Bench warned.

Officials termed today's action "anticipatory," guarding against the future possibility of some American bank geeting into trouble if a foreign country defaults on a loan. In such a case, the comptroller's office, responsible for supervising national bank lending, might have been challenged for supervisory procedures that were too casual or imprecise.

Staff members of the Senate Subcommittee on Foreign Economic Policy, which last September issued a widely publicized report warning that American and other multinational banks were becoming overextended, welcomed Heimann's action as a positive."

They pointed out that the private bank debt assumed by the less-developed countries grew by $3 billion more in 1977 than in 1976, with Mexico and Brazil still leading the way as major borrowers.

Among countries thought to have the most pressing repayment problems are Zaire, Peru, Turkey, and Portugal. Heimann said that his action, which has been filed by the Federal Register, was not prompted by any known problem of any individual bank or individual country.

"I don't think it will require a change in [any bank's] existing activities, but I'm not really sure," he said. The practical effect, he added, is that a bank which is determined to have exceeded the 10 per cent limit will be required to reduce the total, or show that there is more than one borrower, each with "the financial means to service their own debt obligations."

If the bank can't prove the "means" test, then its loans to all agencies within the government must be "aggregated," and not total more than 10 per cent.

Officials refused to say how the newly proposed rule would have worked in the notorious Petromina case of two years ago. Petromina, the national oil company of Indonesia, defaulted on bank loans, which were eventually covered by the state of Indonesia.

An official of the comptroller's office said that in any state-owned agency, the government is the creditor of last resort, but that doesn't necessarily mean that the loans to such an agency have to be aggregated. "The ownership is not the test," the official said. "The test is whether the entity itself has the means to pay off the loan."

Interested parties can comment on the proposed ruling for 60 days after publication in the Federal Register.