Mark McGwire played first base for the Oakland Athletics and St. Louis Cardinals. In 1987, he broke the single-season home run record for rookies, with 49. In 1998, he broke the single-season home run record by hitting 70, (later eclipsed by Barry Bonds with 73 in 2001). For his career, he averaged a home run once every 10.61 at bats, the lowest at bats per home run ratio in baseball history (Babe Ruth is second at 11.76). Ad from 2/97.