Objective: To explore the relationship between individual factors (i.e. affect, self-efficacy, and self-regulation), social and environmental factors, and their effects on the level of participation in physical activity (PA). Design: Undergraduate and graduate students (N = 386) completed 11 online measures assessing physical activity level and reactions to physical activity participation at Time 1, 9 online measures at Time 2, and a measure of physical activity participation at Time 3. Measures included those assessing affective reactions to PA, outcome expectancy, self-efficacy, self-regulation, social support, and perceptions of the environment. Results: Affect had a small total effect on METs (Ã =.13, p=.03), which was partially mediated by self-regulation, a strong predictor of METs (Ã =.45, p<.01). The total effect of affect on METs was substantially reduced (Ã =.05, p=.34) when self-efficacy was added as a precursor in the model. Self-efficacy influenced both METs (Ã =.39, p<.01) and affect (Ã =.23, p<.01). Adding environment and social support as predictors of self-efficacy (Ã =.23, p<.01; Ã =.19, p<.01, respectively) further reduced the influence of affect on METs (Ã =.03, p=.63) as environment and social support influenced affect (Ã =.20, p<.01; Ã =.14, p=.02, respectively) and METs (Ã =.15, p=.02; Ã =.21, p<.01, respectively). Conclusion: As in earlier studies of acute affective response to PA, these results provide evidence that anticipatory affect is positively associated with behavioral decision-making related to PA participation. Although increasing an individual's self-efficacy for PA should increase their affective association with the behavior, affect may not influence PA decision-making independently of self-efficacy and ecological factors (i.e. environment and social support).