Within a few years, it will cost more to pay the interest on America's debts than it does to defend the country.

By the end of this week, the government will owe $3,195,000,000,000. That's $3.195 trillion - amounting to about $13,000 for every man, woman and child - and the figure is rising daily.Congress and the Bush administration are now struggling to reduce the size of the budget deficit, which each year adds to a mountain of debt that has tripled in the past 10 years alone.

But the fact remains that debt service costs - interest payments to lenders for all the money the U.S. government has borrowed - are getting so large that they will soon surpass the defense budget.

It will be 1996, at the earliest, before the government will be in a position to pay off any of its debts and so reduce the interest payments.

The latest government spending plan calls for the United States to spend about $290 billion a year on defense for the next three years. Though subject to some revision, possibly downward, the Pentagon's budget likely will stay close to that target.

According to the White House's midyear review of the economy, interest outlays on the public debt are expected to be $260 billion in 1990, rising to $277 billion in 1991 and $293 billion in 1992. In 1993, interest payments will be $309 billion.

"It's mind-boggling really," says economist David Jones of Aubrey G. Lanston and Co. in New York. "And there's no give-and-take on the debt costs because that's the one area that cannot be cut or else we're bankrupt."

But other things can be cut - and taxes raised - to eat into the deficit. The heated budget debate that has been dominating Congress for the past couple of weeks is about just what those things should be.

Prime contenders for cuts to feed the debt demand have included spending on defense, farm subsidy programs and money for Medicare. Gasoline, high-priced luxury goods, cigarettes and alcohol have been targets for higher taxes.