How to keep a delicate world banking system stable

''Concerned? With so many banks heavily exposed and some debtor nations acting irresponsibly?

''If I told you I wasn't concerned, I'd be lying.'' Speaking is a top United States banking source, with a broad overview of the international banking system.

''We cannot afford another shock to the system like Mexico,'' says another expert, Richard S. Dale of the Brookings Institution. ''The (rescue) package for Mexico is very late'' and has caused ''an enormous loss of confidence'' in the system.

Both men are talking about the huge burden of debt - a manageable $100 billion in 1973, approaching $600 billion now - among developing nations, some of whom cannot pay back their loans.

To avoid default by any single nation or a combination of debtors, governments, central banks, international lending agencies, and private banks are cooperating in a massive effort to keep the system stable.

Currently this effort centers on rescheduling or stretching out debt repayments by two of the most troubled nations, Poland and Mexico. If the bail out strategy works, experts hope to avoid the kind of default that would threaten the stability of the international banking system.

More than half of all money borrowed by poor countries is owed to private Western banks - primarily US banks in Latin America and European banks in Eastern Europe.

Poland, for example, owes money to more than 500 banks - all of which must act in concert in agreeing to stretch out repayments. The great effort now is to prevent any single nation from declaring default - the inability to pay anything , in effect wiping its slate clean of debt.

''If that happens,'' says a Federal Reserve Board official, ''the loans involved are classified as a loss, or bad loans. Creditor banks no longer can carry them on their books as assets. So their assets and capital shrink.''

Then, says Mr. Dale, comes the ''real danger of a defection of depositor confidence.'' Savers, alarmed by what they hear of their bank's exposure to loss , may pull out their savings. A run on the bank ensues.

Small or medium banks, experts say - with which savers feel a sense of personal connection - are more likely to suffer this kind of confidence loss than giants like Bank of America, Chase Manhattan, or Citibank.

It is, on the other hand, the giants that are most heavily exposed to the threat of international debts going sour. Some major US banks have billions of dollars sunk in Mexico, Argentina, and Brazil.

All this explains the enormous effort the interlocked financial community is making to keep Poland and Mexico afloat - the first with $26 billion of debt, Mexico with $80 billion.

The effort includes not only hundreds of individual creditor banks, but also the Federal Reserve and other central banks, plus international agencies like the International Monetary Fund (IMF).

The Mexican rescue package includes a $1 billion cash advance from the US to Mexico for future deliveries of oil, another $1 billion in credits for US farm exports, and a large Federal Reserve contribution to a $1.85 billion loan put together by the Bank for International Settlements in Switzerland.

All this is a prelude to an IMF loan of $4 billion to $5 billion. The IMF requires strict ''conditionality'' - that is, austerity measures by the Mexican government to keep its debt from growing. Spending beyond its means - on the assumption that the price of oil would continue to go up - led to Mexico's current troubles.

Outgoing President Jose Lopez Portillo so far has not grasped the nettle of austerity. Bankers await the advent of incoming President Miguel de la Madrid Hurtado, hopeful that he will accept the IMF judgment on to what has to be done.

Assuming that he does, and that Mexico and Poland limp along with some interest payments, the focus may shift to Argentina - also unable to pay principal on its $36.6 billion debt.

Brazil, the largest debtor of all with over $80 billion owed, has prudently cut back spending. Until now the Brazilians have met interest payments and, having watched Mexico teeter on the brink, appear determined to avoid the abyss.

Beyond this short-term scrambling, says C. Fred Bergsten, lies the problem of what happens if banks, grown suddenly cautious, pull back on their lending to developing countries in general.

''The big banks,'' says Mr. Bergsten, director of the Institute for International Economics in Washington, ''cannot back out precipitously from their loans, lest they send the debtor countries straight into default and lose their loan money.

''But already,'' says Bergsten, formerly a high US Treasury official, ''there are sharp cutbacks by banks in granting additional loans. This constrains the economic growth rate of developing countries and further ratchets down world trade.''

Mr. Dale cites the example of smaller banks and Japanese trading houses, now refusing to renew loans to Brazil.

For poor countries without oil to sell, the problem is severe. High interest rates add to their debt payment burden. Because of worldwide recession, they sell fewer goods of their own. This means they can buy less from other lands.

Americans are directly affected by this cycle. Forty percent of all US goods sold abroad go to developing countries - more than the US sells to Europe and Japan combined.

From all this, two general points emerge:

* A domino effect of default among debtor nations must be avoided, if the international banking system is to escape disaster.

* Loans to developing nations should be prudently trimmed but not cut back so sharply that, willy-nilly, debtors cannot repay old loans and are pushed into default.