Reserve guard Fred Brown and Lonnie Shelton each scored 20 points to lead the Seattle SuperSonics to a 102-90 National Basketball Association victory over the Bucks yesterday in Milwaukee.

Center Jack Sikma had a basket, two free throws and a rebound in the final 3 1/2 minutes for the Sonics (6-0). The Bucks (4-2) were led by Sidney Moncrief with 20 points.

Brown played 26 minutes, compared with an average of 16.6 in previous games, and made nine of 15 shots, including a three-pointer. He replaced Gus Williams, who had the flu.

Said his coach, Len Wilkens: "Our guards are much deeper than last year. Freddie is a smart player who gives us what we need and gives us instant offense."

The Bucks, with Bob Lanier and Marques Johnson each scoring seven points, took a 22-10 lead as Seattle missed several layups in the first period.

With his team foundering, Wilkens made a change that turned the game in Seattle's favor. He put James Donaldson, a 7-foot-2, 275-pound center, into the game, and almost immediately Donaldson shut off Milwaukee's inside game. The Sonics also began pressing Milwaukee's guards, and outscored the Bucks, 25-8, to take a 52-40 lead near the end of the second quarter.

Milwaukee's four guards missed 23 of 39 shots.