Predictors of changes in three measures of physical activity over 1 year were examined in a community sample of 1,411 California adults. Five percent of women and 11% of men adopted vigorous activities (e.g., running), and 26% of men and 34% of women adopted regular moderate activity (e.g., walking). About 50% of vigorous exercisers and 25-35% of moderate exercisers dropped out in 1 year. About 9% reported large 1-year increases in globally rated activity level, while about 7% reported decreases in global activity. In multivariate analyses, adoption of vigorous activity was predicted by young age, male gender, and self-efficacy. Maintenance of vigorous activity was predicted by attitudes toward physical activity. Adoption of moderate activity was predicted by health knowledge, and maintenance was predicted by specific exercise knowledge, female gender, and self-efficacy.