This study investigated the influence of affect on individuals' intentions to engage in physical activities such as exercise. Behavior intentions were examined through the Theory of Planned Behavior (TPB).An experimental survey was conducted among 153 undergraduates randomly assigned to three conditions – positive affect, neutral, and negative affect. Key variables from the TPB were assessed across these conditions.Analyses showed that participants in the positive affect and the negative affect conditions reported lower intentions to exercise than those in the neutral condition. Participants in the negative affect condition also reported more unfavorable attitudes toward exercise than their positive or neutral counterparts. Other TPB measures remained stable across the three conditions. In particular, perceived behavioral control and attitude were significant predictors of behavior intention in the pooled sample.These results underline the important role that affect, especially negative affect, plays in individuals' decision to exercise. Rational models for health behavior change, such as the TPB, should take into account the impact of affect.