Based on Solomon's Opponent-Process theory (1980), it was predicted that individuals involved in a regimen of regular aerobic exercise (active; n = 18) would respond to an acute bout of exercise with reduced negative and/or increased positive affect compared to nonactive counterparts (nonactive; n = 12). State Anxiety (SA), positive affect (PA), negative affect (NA), and self-reported fatigue were assessed immediately prior to, every 6 min during, and every 6 min following a 24 min bout of bicycle exercise performed at an RPE of 13 (+/- 1). As expected, no significant group differences occurred for RPE (M = 13.5 for nonactive, 13.2 for active). The active group did, however, exercise at a greater absolute workload than the nonactive group (261.0 +/- 22.4 W vs 200.0 +/- 19.98 W, respectively). Analyses indicated similar changes in SA and fatigue for both groups, with significant reductions in SA occurring at 6 min post-exercise and remaining below pre-exercise levels throughout the post-exercise period, while fatigue was reduced at 12, 18, and 24 min post-exercise. A significant Group x Time interaction occurred for affective valence (PA - NA; p < .01). Post hoc analyses indicated that for the active group, affect increased modestly (i.e., more PA, less NA) during exercise; this increase was sustained post-exercise. The nonactive group evidenced a sharp drop in affect (i.e., less PA, more NA) during exercise followed by a small post-exercise rise which did not return to pre-exercise levels. The results of the present study partially support the Opponent-Process model as an explanation for exercise-related affect. Although there was no differential anxiety response as a function of activity level as the model would predict, there was a differential response for affective valence in accordance with predictions.