Last year, the world's developing countries owed U.S. banks close to $90 billion, almost three times as much as in 1975 when the Federal Reserve began to keep track of overseas lending on a country-by-country basis.

Yet despite the well-publicized problems involving loans to Poland and Romania, there appears to be little concern of a banking collapse as a result of overseas lending. In fact, some experts are more concerned about the impact on the world economy if banks cut back their lending than about the effect on banks if they don't.

For one thing, debtor countries will do almost anything to avoid defaulting on their loans, experts believe. Moreover, major commercial banks are sufficiently diversified and strong enough to withstand small losses on parts of their business, and even if a smaller bank were to get into trouble it would likely be bailed out by the central banks of the industrialized world.

Finally, U.S. banks don't have much money at risk in countries that seem to be in the shakiest financial condition.

Nevertheless, there is growing concern about international banking. There's "no question" that the system faces more difficulties now than a few years ago, according to one top New York banker.

Developing countries that ran up large debts in the 1970s because of the sharp run-up in oil prices may have difficulties making payments because of depressed exports and low prices for their commodities. More of them are likely to ask banks to renegotiate their debts on easier terms in the next few years, bankers agree.

This could force a few small banks out of business, one World Bank official said last week. High profits have attracted hundreds of banks into the international market in the last decade, and as the profits shrink it is "very much a possibility that poorly run banks will go out of business," he said.

But the official isn't greatly worried by that prospect: if banks make poor loans there is no reason why they should not suffer, and no reason why the entire financial system should be threatened, he said.

While bankers don't expect a default, they do warn that the Reagan administration may force a default for political purposes, as some officials--including Defense Secretary Caspar Weinberger--reportedly favored in the case of Poland. (Under terms of some bank loans, they automatically are classified as in default if the borrower defaults on other loans, such as those made by the U.S. government.)

Bankers prefer to ease loan conditions to declaring default, which would destroy confidence in the whole edifice on which international lending depends.

According to the latest Federal Reserve figures available, U.S. banks last September were owed close to $320 billion in direct loans to foreign countries. Of this:

* $198 billion was loaned to other industrialized countries, most of which are hardly candidates for default;

* $23 billion was in OPEC countries, including Venezuela and Ecuador;

* $61 billion in the rest of Latin America and the Caribbean, with the bulk of that concentrated in the bigger, stronger economies;

* $25 billion in Asian developing countries, excluding oil-exporting Indonesia, with South Korea--the biggest borrower--owing $8.5 billion;

* $8 billion in Eastern Europe;

* $4 billion in Africa (excluding Liberia and the oil exporting countries), where the world's poorest and weakest nations are concentrated. One-fourth of this was lent to Egypt, which also is heavily supported by official U.S. aid.

Another $45 billion to $60 billion has been borrowed from U.S. banks via off-shore banking centers, such as the Bahamas and the Cayman Islands, according to Federal Reserve figures. Regulators here cannot trace the final destination of this money.

Loans by U.S. banks to Eastern Europe have not grown much in recent years. "New money flows to Comecon countries have now more or less come to a halt," one Federal Reserve official said last week.

And U.S. bankers also have pulled out of much of Central America--one area where many expect nations to have difficulties meeting their debt obligations. American "banks have not got much exposure down there" now, the Federal Reserve official said. However, Costa Rica, which owed a total of about $600 million to all American banks last June, is now negotiating a stretched-out repayment schedule for some of it with a group of banks headed by Bank of America. Nicaragua already has rescheduled its commercial bank debts.

Many of the countries on the international casualty list are simply too poor to have attracted much commercial bank money, and rely almost exclusively on official government aid or loans from international institutions, such as the World Bank and International Monetary Fund. One major bank that does business with about 120 countries said last week that 62 percent of its loans are to just 13 nations--most likely the 13 largest and most industrialized. The bank asked that it not be identified.

Eight nations were in such difficulties last year that they were forced to reschedule their debts to creditor governments. Seven of these were in Africa, where U.S. banks have traditionally not been big lenders, and the eighth--Pakistan--owed American banks only $161 million last June.

There is some small U.S. involvement in other financially troubled nations. Sudan, considered a "basket case" by some bankers, owed American banks just over $250 million in the middle of last year. Bolivia, also in serious economic trouble, according to one banker who did not want to be named, owed American banks $460 million last year.

But the bulk of U.S. bank lending to developing countries has been to big, stronger economies in Latin America. That area as a whole is "relatively speaking, not so bad," said one New York banker who long has been involved in international banking.

The lending in Latin America is overwhelmingly concentrated in a few countries: Mexico with $20 billion; Brazil, $17.5 billion; Venezuela, $9.2 billion; Argentina, $9.2 billion; and Chile, $5.5 billion.

While some U.S. analysts and bankers were very worried about Brazil's debts, "I think it was just a fear of big numbers," commented one official from the International Monetary Fund. Most bankers remarked that Brazil was perceived as a better risk now than a short while ago.

One senior international banker, who said he was not concerned about the deep indebtedness of Mexico--which has stepped up its borrowing in recent years in anticipation of a large rise in oil earnings--remarked, "It is not going to go out of business." However, others do worry that Mexico could have payment difficulties in the wake of unexpectedly low oil prices unless it scales back its development plans. The key will probably be whether or not banks keep on expanding new credits to Mexico to tide it over a probable "temporary bind."

U.S. bank lending to Chile has soared recently--climbing by more than 70 percent in the year ended last September. Bankers had mixed views about this, with the head of a small bank's international operations commenting that the government is trying "to put its house in order" and others still concerned that some of the loans may have to be stretched out or renegotiated.

Several analysts expressed concerns last week about Peru, which is one of many developing countries being hurt by extraordinarily low commodity prices, which provide export earnings. U.S. banks have close to $2 billion in loans there.

But just as Brazil is no longer regarded with as much concern and Turkey is now in much better shape after virtual bankruptcy, so other countries now being agonized over may be viewed differently next year.

Alarmists have always found reasons to fear that one country or another would be unable to service its debts, commented Citibank's G. J. Clark, who is executive vice president responsible for country evaluation. In retrospect the fears have always been exaggerated, he said.

A lot of the anxiety about banks' exposure in foreign countries comes from those--such as bank regulators--who are paid to worry and to be cautious, said one Federal Reserve official. "When you see Federal Reserve governor Henry Wallich making speeches" about the dangers for U.S. banks in international lending, "why, that's part of his job."

One reason for the frequently expressed fears about overseas lending--which one World Bank official said had amounted to "near hysteria on the part of U.S. regulators" at one stage in the late 1970s--is that international lending is "conceptually" more risky than lending within the United States, one regulator said.

Banks at a distance from their debtors may be less able to gauge what is going on than they would be with a borrower at home, or they may have trouble dealing with a different legal and social structure or different business mores, he said.

Another problem is that "sovereign risk" loans are made to or guaranteed by a nation's government, and if the loans go bad the banks cannot put a lien on the assets of the government as they might in the case of a company at home. There are also fears that an overseas borrower might be unable to service or repay foreign currency debts because of its nation's overall balance of payments problems and lack of foreign exchange, even when the individual borrower is not bankrupt. Political upheaval that could threaten investments is another obvious danger to international lending.

Despite this additional "conceptual risk," the losses on international lending in recent years have actually been only about a third as great--in relation to total loans--as losses on domestic loans, commented analyst Irving Geszel of Bear Stearns and Co. The same is true, he said, for "nonperforming assets," or loans where the banks have changed the terms, perhaps by stretching out the loans, so that they are more advantageous to the borrower and make less money, or no profit at all, for the lender.

However, international lending is clearly losing some of its gloss for the banks. The "spreads" that determine profits--between the cost to banks of borrowing funds and the interest rates charged on the loans they make--are now "razor thin," according to a senior Bank of America official.

While North American banks are generally more sanguine about the future risks of international lending than banks in Europe and Asia, they are more concerned now than they used to be, according to the preliminary results of a worldwide survey of about 200 banks. It showed that banks worry about the expense and problems involved in rescheduling debts rather than a default and chain collapse, reported Geoffrey Bell, executive vice president and director of Schroder International.

Such worries are real. "Sure it's fair to say that no country has defaulted," said the chief of lending at one of the nation's top five banks. "But who would expect any country to do that? Where would it go? As bankers what we should be concerned about is nonperforming assets . . . Do you think a Polish asset is worth a dollar?"

As more countries reschedule debts, there will be more nonperforming assets. There is a dilemma about how banks should react to that. It would be prudent to slow the expansion of international bank lending, said Bell, but not in a "pell-mell" fashion that would exacerbate the difficulties for debtor countries in coping with current interest payments and import needs.

A paper by staff officials of the International Monetary Fund last year said that commercial banks had added "an additional destabilizing element" to debtor countries' cash flow. It found that in six case studies of countries that restructured their debt in the latter part of the 1970s, banks had stepped up lending when times were good, and cut back sharply when times were bad and other sources of badly needed foreign exchange were also drying up.

Romania's current problem is largely a short-term liquidity squeeze brought on by the halt in bank lending to all Eastern European countries when Poland's debt problems mounted, some analysts believe. Other Eastern European nations, such as Hungary, may also start to hurt, even though their economies are well managed, if bankers continue to shun Eastern Europe, warns Jan Vanous of Wharton Econometric Forecasters.

If bankers grow too nervous, or try to reduce their loan commitments in a hurry, then many basically healthy economies may be forced into liquidity crises, he warns. North American banks "seemed very concerned about the effects on the system of abrupt government action or new controls" on lending, Bell said in his report on the survey of banks.

It is for this reason that many of those in the world of international finance worry more about the dangers to the world economy if banks cut back on their overseas lending than the danger to the banks if they do not. The threat in the 1980s is "not that countries will go out and default," but that they may have to depress their economies to cope with less foreign money, and will "contribute to the overall world tendency to deflate" and slow down economic growth, said Nicholas C. Hope of the External Debt Division of the World Bank.

If the resources of the international financial agencies are simultaneously reduced, as the Reagan administration would like, then the problems multiply.

A World economic slowdown will hurt the creditworthiness of overseas countries, but it will also damage domestic borrowers. "If interest rates go to 20 percent" in a world economy that is in recession and burdened with high oil prices "then all bets are off," said one senior New York banker. "But don't just look abroad. The same thing would happen here."