Data from the San Diego Health and Exercise Baseline survey conducted in 1986 were used to contact participants for the follow-up. The difference in reported activity level from time 1 to time 2 served as an estimate of change in activity. The follow-up survey elicited a two-year history of vigorous exercise and moderate-intensity exercise, both of which have substantial health benefits, as well as different determinants. Potential predictors were assessed at baseline and included demographic variables and indices based on social learning theory. Multiple regression analyses determine the combined and independent relationships of such variables as self-efficacy, social support, perceived barriers and benefits and environmental factors on both types of physical activity change. A second analysis separated subjects into the baseline physical activity groups of low, intermediate, and high activity and examined prediction of change versus no-change in each category.