It was one of those electric moments a U.S. senator dreams about but rarely encounters, when he can pounce on an elusive bureaucrat, sink the fangs and chew an answer loose.

Sen. Mark O. Hatfield (R-Ore.), a man with a keen eye for numbers, was pouncing. His quarry, Dr. Donald Fredrickson, sat across the baize-covered table and sifted through his papers.

A year ago, Fredrickson, director of the National Institutes of Health, had been there pleading with Congress to give NIH more money for its biomedical research programs, direly needed, he had said. Now he was trying to explain why he had used some of that new money instead to remodel the clinical center on the big NIH campus in Bethesda. Hatfield said he was shocked, and he was not kidding.

"I want to give you every penny you can use on programs . . . We've bled for them and bled for them. And now we find you can transfer this money, that there is this fat in the budget," Hatfield began. "But we'll get to that . . . ."

Fredrickson nodded a bit at Hatfield, perhaps secure in knowing that senators do not have very long attention spans, or that bigger issues would worry Hatfield before this budget was enacted.

An NIH staff assistant caught the flavor of this pas de deux. On his lap was an open portfolio showing a roster of senators, party affiliation, university ties, time in office -- a dossier to identify friend and foe.

He wrote a note on his yellow legal pad: "Fredrickson -- Shuck and Jive." Score one for Hatfield; he had put his man on the defensive.

On three sides of the I-shaped table in Room S-128 of the Capitol, Frerickson was surrounded by colleagues from the 11 institutes at NIH, the world's foremost medical research center.

They had come before the Senate Appropriations subcommittee for LABOR-HEW to justify their budget, the $3.6 billion that President Carter had requested for NIH for fiscal 1981, the year starting next Oct. 1.

Their appearance was partly ritual; the heads of every federal agency go before both House and Senate Appropriations subcommittees every year.The humdrum sessions drone on for weeks, most of them scarcely noticed by press or public.

Yet nothing is more basic in the government. It is set forth in Article I, section 9 of the Constitution: "No money shall be drawn from the Treasury, but in consequence of appropriations mad by law." And it is in this appropriation process more than any other that Congress ultimately decides what government will do.

In S-128 last week, the issues were of the most basic kind: what diseases the government would attack in the year ahead through grants for research, how hard it would attack each one and, to some extent, how.

Frederickson and the senators were picking their way through the mazes of numbers and tables the appropriations process requires. But the real gist of the process was the drama of biomedical research discoveries at NIH. g

The vast federal investment in medical research has created what Frederickson calls "a time of extraordinary opportunity . . . . A golden age of biomedical research discoveries . . . . A whole host of magic."

His terms seemed appropriate when the institute directors began explaining their progress since last year. They were talking about breakthroughs in such fields as cataract and glaucoma treatment, blood-clot control, burn-shock therapy, vaccines for malaria and rubella. A whole host of magic, as Fredrickson said.

The role of Congress, of course, is to consider the scientists' priorities, perhaps set some of it own, dole out the money and see that the taxpayer gets an adequate return on his investment.

But always there is a problem of who knows best. Hatfield put it this way to Fredrickson the other day: "As laymen, we may be out in left field . . . The legislative branch has a right to elicit your professional judgement, even if it might be in conflict with the administration."

So in a sense these Senate hearings, to be repeated at month's end by a House appropriations subcommittee, are a part of the theater of government. They are a vital step in the process, but only in a passing way.

Budget officials at NIH began working on the fiscal 1981 proposals a year ago. Those figures filtered up through NIH, then through the Public Health Service and the Department of Health, Education and Welfare. After going through hoops at the White House, they went to Capitol Hill.

At each point, questions were raised and challenges issued, so the Senate interrogations last week were rather like midstream inquiries. Later on, after the NIH witnesses have finished, the subcommittees will hear from the public -- individuals and interest groups that will, if the past is a guide, have a large influence on the final budget.

At that stage, the two subcommittees, working independently, will draw up their recommendations. Those in turn will be discussed, and probably altered somewhat, by the full Appropriations committees of the House and Senate.

After each chamber has debated and amended its committee's final bill for NIH, conferees from the House and Senate will meet to work out the differences. These conference sessions often are bitter and disputatious.

When the conferees met last year to discuss the NIH appropriation, it was as easy as pie for the first time anyone could remember. Sen. Warren G. Magnuson (D-Wash.), leader of the Senate delegation, simply went in and told the House group: "We'll accept your figures if you accept ours. We'll take the high figure in each case."

The conference was settled in minutes -- a rarity in the overall appropriations process -- and Congress approved a figure for NIH that was about $250 million more than President Carter had proposed.

That reflected another political reality of the budgetary process. Congress habitually has appropriated more for NIH than presidents have sought -- so habitually, in fact, that it has become a part of a game that White Houses have played for years with the legislative branch.

A president makes political hay be proposing budget cuts. He and his Office of Management and Budget know that in many instances Congress will restore funds to the most popular programs whose constituencies know how to pressure the legislators.

The new budget proposal for NIH is a little different, however, and it is a direct relationship to the apparent harmony of last year's congressional conferees. Those elements will be important as the fiscal 1981 appropriation moves through the legislative pipeline.

Carter has sent to Congress the first NIH budget in eight years that proposes a substantial increase -- up $139 million from last year's appropriation. It is a boost of about 4.5 percent, far from enough to keep the NIH in step with inflation, but still a boost.

Carter, partly at the urging of his science adviser, Dr. Frank Press, wants the NIH increase devoted to more basic biomedical research. This is in keeping with a government-wide commitment by Carter to increased research.

The House-Senate conferees were in agreement last year on dollars, but they did not agree on an important policy issue: How much of the new appropriation should go to basic research and how much should go to programs to train researchers.

The House wanted the new money to go to basic research. The Senate was just as insistent that the money be balanced between research and training. One, Magnuson and Hatfield insisted, could not get alone without the other.

That disagreement resulted in the conferees backing away from the issue.

They put no specific language in their final report to instruct NIH on how the new money should be spent.

NIH, admittedly, was in a dilemma. But Fredrickson decided to pump the money into basic research -- in effect, disregarding the Senate's views. That decision came back to haunt the cool, articulate NIH director last week when he went before the Senate subcommittee.

Not only did Fredrickson put the current fiscal year increase into basic research, but Carter's proposed budget puts more in the same area while cutting back on the training grants that are vital to many medical schools and hospitals.

Sen. Thomas F. Eagleton (D-Mo.) and other senators jumped on Fredrickson with both feet. Eagleton was visibly bothered by the development. He and others felt the NIH director had blithely ignored the Senate's wishes on training grants.

The handwriting was quickly on the wall for the fiscal 1981 budget: The dispute between Congress, the amdinistration and NIH over the use of the new money will be a major sticking point in this year's process.

"We read very carefully and we try as best as we can to comply," Fredrickson told Eagleton, "Sometimes it puts us in a very difficult position."

Eagleton wasn't impressed. "We can ask you to do the impossible," he told Frederickson. Then, with a twist of irony: "Gov. [John] Connally says we can increase defense sepnding, cut taxes and balance the budget. So we could ask you to do the impossible."

One thing seemed certain from all of this. The senators were not pleased with the answers and they didn't like seeing their instructions swept aside. Fredrickson would be hearing more about this, for the show had just begun.