McGwire, batting second and listed as the second baseman, took a called third strike from Milwaukee's Jeff D'Amico in the first inning. As planned, McGwire then left the game.
Cardinals manager Tony La Russa wanted McGwire, who was activated from the disabled list before the game, to get a taste of live pitching.
McGwire was placed on the 15-day disabled list July 13, retroactive to July 7, because of patella tendinitis in his right knee. He is still unable to play first base, his usual position.
The NL Central-leading Cardinals hope to use McGwire as a pinch-hitter in September. It's uncertain whether he will be able to return to the lineup on a full-time basis if St. Louis reaches the postseason.
McGwire has hit 30 home runs despite missing 70 of the Cardinals' 140 games. He took part in batting practice and crushed at least 10 balls over the wall in left field as his teammates stopped and watched.
After J.D. Drew led off the game with a single, McGwire stepped to the plate and was greeted by cheers from the sparse crowd.
McGwire worked the count to 1-2 before taking a called third strike. He fouled off three pitches.
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