Maj. Arthur D. Nicholson Jr., who was shot to death by a Soviet guard in East Germany, was portrayed yesterday as one of the Army's best and brightest Russian specialists who had made daring missions behind the Iron Curtain to gather intelligence on the Red Army.

He was killed while he was engaged in a daylight mission in a clearly marked U.S. vehicle under an agreement in effect since 1947, the State Department said.

Prof. Jiri Valenta of the U.S. Naval Post Graduate School in Monterey, Calif., said Nicholson studied under him for two years and was one of the brightest students he has had in 10 years of teaching there.

Nicholson, who was 37 at the time of his death, was one of only nine students in his field who wrote such a distinguished paper during that 10-year period that the Navy school published it, Valenta said. Nicholson's topic for his master's thesis was U.S.-Soviet relations, with the different approaches to arms control a featured part of it.

Valenta, who left Czechoslovakia in 1968 to teach in the United States, said Nicholson spoke and read Russian well, but not expertly, and followed the Soviet debate over nuclear arms in Russian military journals.

"He was very objective," said Valenta. "He did not accept either concept of left or right but tried to understand the relationship between the United States and Soviet Union when it came to nuclear arms. He had no illusions about the Soviet Union, but told me things had become so dangerous that it was important to find a way to negotiate" arms control agreements.

Such objectivity and commitment makes the loss of Nicholson "particularly tragic," Valenta said. Nicholson graduated from the Navy school at Monterey in 1980 with a master's degree in international relations and a major in Soviet studies. He went on to study Russian at the nearby Defense Language Institute and planned to pursue a doctorate in Soviet studies, according to Valenta.

Nicholson had been attached to the U.S. Military Liaison Mission in Potsdam, West Germany, since February 1982. Pentagon officials refused to concede that the mission was a spy outfit, pointing out that its monitoring behind the Iron Curtain is done openly. At times, however, the monitoring becomes a risky cat-and-mouse game, according to one former member of the group.

"We'd go in at 90 miles per hour between 11 at night and 1 in the morning to try to keep the Russians from seeing where we were going," said the former mission member who did not want his name used. "It's a very dangerous job."

Before reporting to Potsdam, Nicholson had served at the Army's intelligence facility outside Washington at Arlington Hall from 1974 to 1975 and then went to the Military Intelligence Detachment at Munich, West Germany.

Nicholson, son of a retired Navy commander, Arthur D. Nicholson Sr., moved from McLean, Va., to Redding, Conn., and attended his last two years of high school there, graduating in 1965. He went into the Army after graduating with a bachelor's degree in philosophy from Transylvania University in Lexington, Ky., in 1969.

In his Redding High School class of about 115 students, Nicholson was one of the few who expressed an interest in a military career, according to his high school guidance counselor, Richard E. Roth.

"To most kids, military was a semi-dirty word at that time," Roth recalled, but added that no one ever teased Nicholson about his ambition. "He was a big, good-looking boy with curly blond hair. He was outgoing and liked by his classmates and the faculty."

Thomas Lalley, a friend of Nicholson's parents in Redding, said that they were "very proud of him, very pleased with his career. He was a fine, upstanding young American."

Another neighbor, David Peterson, said the Nicholsons were proud of their son and their daughter, Cathy, a flight attendant. "They'd brag about their son and daughter and they had reason to," he said.

Nicholson is also survived by his wife, Karyn, and a daughter, Jennifer, 9, who live in Germany.