Physical activity is a recognized preventive health measure for seniors and an important focus for senior centers. This paper employs the Andersen Behavioral Model to explore increased physical activity and participation in three types of senior center activities: physical fitness, dance/aerobic classes, and chair exercises. Data were collected in 2006 on 798 and in 2007 on 742 participants at 21 multipurpose senior centers in a large urban county. Logistic regression analysis (PROC RLOGIST in SAS-callable SUDAAN) was employed to predict increased physical activity, with modes of center participation in physical activity as mediating factors. Predisposing and enabling factors predicted both engaging in center-based exercise programs and increases in physical activity; but the strongest predictors of increases in physical activity were needed factors: physician recommendations to increase exercise and to lose weight. Implications are that both SCs and healthcare providers are important to promote physical activity in the older population.