IN THE SPOTLIGHT. Did power change the admiral?

As a midshipman at the United States Naval Academy in the late 1950s, John Poindexter could measure his success every six weeks, when academic rankings were posted. Smart, disciplined, and motivated, ``Poins'' by all accounts thrived on the academy's fierce classroom competition. With rivals at his heels, in the end he raced triumphantly through his exam in electrical engineering (a subject called ``skinny'' in the impenetrable midshipman slang) and finished first in the Annapolis Class of 1958. His achievement was more remarkable for the fact that in the winter term of his last year he was a ``six-striper,'' or brigade commander. Equivalent to student-body presidents, such leaders have a heavy burden of duties both ceremonial and substantive. ``Nothing was unusual for our record-setting six-striper,'' said an admiring yearbook inscription.

Little did they know. John Poindexter, having risen through the ranks to become a vice-admiral and President Reagan's national-security adviser, is now sitting before congressional investigators to explain how he abetted the sale of American arms to Iran, and subsequent diversion of profits to the contras in Nicaragua.

He is the man at whom all fingers are pointing. Above him, President Reagan says he had no idea what was going on. Below him, Lt. Col. Oliver North, his former National Security Council subordinate, claims everything he did was authorized.

In his dramatic first hours of testimony, Admiral Poindexter said both these assertions are right, that he gave the go-ahead for Colonel North's actions, and at the same time decided not to inform the President. This insulation was to provide ``some measure of deniability if it ever leaked out,'' Poindexter said.

The Poindexter who appeared before Congress to say these things is a man who has been changed, if not corrupted, by power, some associates say. In the space of five years at the White House he went from being an assistant-to-an-assistant, to head of the NSC, and wielded an influence in national-security affairs that even admirals only dream of.

He helped direct the capture of the Achille Lauro hijackers while eating lunch at his desk; he flew to Iceland and took part in superpower negotiations that touched, however briefly, on the elimination of nuclear weapons from the world.

He began sounding like a politician, says one officer who has long known him, denigrating the military chain of command and talking of high officials whom he couldn't work with. ``It can happen to good people,'' says this officer. ``They get over there where the air is thin.''

It may have been an abrupt transformation for someone who had been a classic example of small-town-USA values, almost a Jimmy Stewart movie character come to life.

Poindexter grew up where the land is flat and the sea is far, as do a surprisingly high percentage of Navy officers. In his case the place was Odon, a hamlet in southwest Indiana near nothing in particular. A nearby town is named Plainville.

He was apparently a grave and meticulous boy who showed promise to adults in a manner that did not overly offend his peers. He went camping. He worked at the local movie theater.

His father, a banker, wanted him to attend a service academy. As an Eagle Scout and top student, John won an appointment to the Naval Academy from Republican Sen. Homer Capehart.

Some Annapolis plebes are bitter, feeling forced into the place by domineering parents. Others simply can't take the constant ranking and pressure. Poindexter loved it. It was a world in which he, always good with paper and books, could excel.

``He was a fine midshipman, a pretty smart gentleman,'' says retired Comdr. William Chipman, who in 1958 was a lieutenant and Poindexter's company officer.

He studied hard, but not so hard as to be unpopular, according to classmates. He was on the debate team. He dated a University of Maryland student who became his wife.

Most of all he transformed his life through academics. It is hard to exaggerate how much finishing first in an Annapolis class affects an officer's Navy career. Ever afterward, Poindexter was a man thought to be on a path that leads to the position of chief of naval operations, the Navy's top officer and a member of the Joint Chiefs of Staff.

After tours at sea and time at Cal Tech for a doctorate in nuclear physics, then-Lt. Com. Poindexter landed a coveted staff job in the office of the secretary of defense, Robert McNamara. He did systems analysis of Navy personnel assignments.

``He was instrumental in seeing that the Navy got more efficient use of manpower,'' says retired Rear Adm. Clarence Hill Jr., who is now managing a legal defense fund for his embattled former colleague.

That is the sort of thing Poindexter is remembered for - rationalizing this process, streamlining that one, computerizing a third. Through a series of increasingly important jobs in the Pentagon he ``handled the paper flow,'' in the words of one associate. Reportedly, his top service medal, the Legion of Merit, was awarded for correspondence and computer work.

It was for such a service that he was brought to the White House. Richard Allen, Reagan's first national-security adviser, hired Poindexter in 1981 as military assistant to the assistant NSC director. His job, said Mr. Allen in a recent breakfast with reporters, was ``to do whatever liaison work needed to be done on a mechanical, administrative level with the Pentagon. He was not an analyst of foreign policy.''

Something changed. Robert McFarlane, an ex-marine, eventually rose to the top NSC job; he had been a year behind Poindexter at Annapolis and admired him. When Mr. McFarlane in turn resigned from government service, he recommended Poindexter as his successor. In late 1985, the by now vice-admiral took the job. He became known primarily for his aversion to interviews, and for being a guiding force behind a plan to mislead Libyan leader Muammar Qaddafi by planting disinformation in the press.

Then events followed that are now well known. Arms were sold, funds were diverted, Congress was lied to, a cover-up was tried. Poindexter resigned and accepted a reduction in rank to rear admiral.

The question now, say Iran-contra committee sources, is what sort of man he is. Did he really approve these actions on his own, shielding the President? That might go against the whole pattern of his career as a loyal staff officer.

Sen. John Chafee (R) of Rhode Island, secretary of the Navy under President Nixon, employed Poindexter on his staff. ``I always found him to be nothing other than an honorable man,'' Senator Chafee says.

He adds that Poindexter was ``not a renegade, not likely to do things without informing his superiors.''

Many of the admiral's friends say that if he claims he is responsible then they believe him. Naval Reserve Lt. Comdr. Gordon Schaaf, an Annapolis roommate, had lunch with Poindexter recently. He says they talked of many things - kids, wives, plans. They touched on political troubles only lightly. ``He was confident he had done the right thing,'' Mr. Schaaf says.