After 16 years of pleading its case before six tribunals, a small Oregon bus line won its long antitrust battle against Greyhound Corp. yesterday in the Supreme Court.

In refusing to hear an appeal by Greyhound, the high court upheld a February appeal court decision ordering Greyhound to pay $23 million in damages to Mt. Hood Stages of Bend, Ore.

The outcome was a personal victory for William J. Niskanen, a self-described "tough Finn" and former boxer who founded Mt. Hood Stages back in the 1920s. In a long string of cases leading up to yesterday's decision, Niskanen showed how in the 1950s Greyhound sought to drive Mt. Hood Stages out of business so it could take over its routes.

Niskanen, now in his mid-70s, was playing golf yesterday and could not be reached for comment. His attorney in the case, Eugene C. Crew, said in San Francisco: "While it is unfortunate that our client's battle against the largest bus carrier in the nation took 16 years to win, today's decision proves that courts will grant full restitution to the small competitor who perseveres."

A spokesman for Greyhound said the company had no comment.

Between 1947 and 1954, Greyhound took over eight small lines, completely surrounding Mt. Hood Stage's routes. When Niskanen protested to the Interstate Commerce Commission that his future could be in jeopardy, Greyhound agreed to "maintain a policy of open gateways."

It was not until the late 1960s, after extensive discovery for his antitrust case against Greyhound, that Niskanen learned what actually happened. The Greyhound documents showed that by controlling the bus lines that fed into Mt. Hood Stages, the giant carrier was able to route unknowing passengers around the Oregon line. Passengers often were taken hundreds of miles out of their way -- and charged as much as $40 a person for the detour.

In 1964, suspecting that his line was being squeezed by Greyhound, Niskanan petitioned to have the ICC stop the bigger line's predatory practices. The ICC enjoined Greyhound, but the company ignored the order. pThen, in 1973, a U.S. District Court in Chicago found Greyhound in criminal contempt of the ICC's order. Greyhound was fined $500,000 and put on 5 years probation.