I.I. Rabi, 89, a Nobel Prize-winning physicist whose activity as a researcher, teacher and adviser to the government made him one of the dominant figures in 20th century American science, died yesterday in his apartment in New York City. He had been ill for some time.

Dr. Rabi, who was brought to this country as a child by his immigrant parents, grew up in Brooklyn, interested himself in science as a small boy reading in the library, and, adopting the discoveries of the quantum revolution, helped lay bare intimate and subtle secrets of atomic structure.

The diminutive, bespectacled Dr. Rabi loved science for its own sake as a matching of wits with nature, but was unenthusiastic about the mechanics involved in constructing experimental apparatus.

He was known for long bouts of whittling while his assistants got his gadgets to work.

His research, including the work on molecular beams and magnetic resonance that won the Nobel Prize in 1944, has been credited with leading to the ultra-precise atomic clocks and to nuclear magnetic resonance imaging techniques that offer modern medicine a powerful diagnostic tool.

A former chairman of the physics department at Columbia University, where he was professor emeritus, Dr. Rabi (whose name was pronounced Robby) played an important role during World War II in the development of radar.

He also made valuable contributions to the Manhattan Project that built the atomic bomb.

After the war, Dr. Rabi, by then one of the elders of American science, headed President Eisenhower's science advisory committee. In that role he warned of great strides made by Soviet science and called for educational improvements here, aimed at producing the "civilized scientist."

A widely read man who combined interests in both science and the humanities, Dr. Rabi often expressed concern about what he considered the inaccurate image of science in the public mind, and asserted that in an atmosphere of scientific secrecy "an informed public opinion can hardly exist."

He argued vigorously for the peaceful uses of nuclear energy.

"Although I have worked very hard" in that area, he said in a lecture in the late 1960s, "I've not been very successful."

Isidor Isaac Rabi was born July 29, 1898, in Rymanov, then part of the Austro-Hungarian Empire. His father, a tailor, brought the family to New York the next year. For a time the family lived above a grocery store the parents ran in the Brownsville section of Brooklyn.

A self-described "kid who read a lot," Dr. Rabi finished all the children's books at his local library by the time he was 11. Then he started on the science books and developed an interest that never flagged.

He received a bachelor's degree in chemistry at Cornell University in 1919 and began graduate work at Cornell two years later. After returning to New York City to continue his advanced studies, he taught at the City College of New York. In 1927 received his doctorate at Columbia University.

After two years in Europe, working under the founding fathers of the quantum revolution, he returned to Columbia, to teach, carry out his own research, and to become one of the principal advocates of the new theories and developments that had transformed physics.

Quantum theory implies that on the atomic scale, certain physical properties are characterized by a set of distinct, permissible values, and that properties such as energy, for example, may not vary continuously, but must jump between allowable values.

The first successes of quantum theory came in accounting for the allowable energy levels occupied by the electrons of the simplest atoms.

In work that pushed far beyond these relatively gross measurements, and the relatively unsophisticated theory behind them, Dr. Rabi developed means for revealing the infinitesimal variations in allowable energies created by magnetic properties of atoms and molecules and their components.

His work was of such precision that it made it possible to distinguish the minute changes in energy of a molecule caused by the interaction of the intrinsic magnetism of the molecule with that of the nuclei of its atoms.

The technique involved the creation of pure beams of molecules and of subjecting them to magnetic fields that could be tuned to the frequencies of subatomic motions identified with molecular magnetism.

Although these investigations proved to have many applications, Dr. Rabi did not pursue them for that reason.

"Science is a great game," he once said. "It is inspiring and refreshing. The playing field is the universe itself."

Survivors include his wife, Helen, two daughters and four grandchildren.