The third biggest item in the $1.45 trillion federal budget next year won't buy a single missile, cover one elderly person's Social Security check or pay any bureaucrat's salary.

Instead, the government will spend about $210 billion in 1992 simply to pay interest to the Americans, foreigners, banks and other lenders from whom it has borrowed money. That's one budget dollar in seven going not to clean the environment or cure AIDS, but to the people and institutions who invest in government bonds.It's an eye-catching figure that rarely comes up when lawmakers debate spending priorities each year, and this year is no exception.

With the Senate opening debate this week on a budget, Democrats are proposing spending more than President Bush on social programs such as education and Medicare and less on science and space.

But there's no dispute that whatever spending plan is agreed to, it will pencil in nearly $210 billion for interest on the nation's debt.

The House approved a fiscal plan last week with $209.8 billion for interest. The budget Bush introduced in February proposed using $206 billion for borrowing. Either way, next year will be the first time the figure will surpass $200 billion as the federal interest payment continues a sharp climb begun during the Reagan years.

"We have become immune to the sound of such enormous numbers, as if they have very little meaning," Rep. Fred Upton, R-Mich., said during the House budget debate.

And as Upton and every member of Congress and the administration know, next year's $210 billion borrowing costs means the government will have that much less to spend for everything else it does.

To help appreciate the enormity of $210 billion, consider:

- Excluding benefit programs like Social Security, Medicare and veterans' pensions, the government will spend $211 billion next year for ALL domestic programs, from running prisons to building roads.

- Individual income taxes owed by Americans in every state west of the Mississippi River totaled $151.2 billion in 1988, the most recent figure available.

- Total U.S. corporate income-tax collections next year are expected to be $102 billion.

- The only costlier programs in the entire federal budget are defense, at $295 billion, and Social Security, at $287 billion.