The present investigation was designed to test the ability of caffeine ingestion to enhance the reduction of body fat with exercise. Mature male rats (average weight of 442 g) were assigned to four grouping combinations of the two treatment variables: caffeine ingestion (caf, no caf) and exercise training (ex, sed). Two groups trained by swimming 90 min, 5 d/wk (caf-ex, no caf-ex) while two groups served as untrained controls (caf-sed, no caf-sed). Forty-five minutes prior to swimming, doses of saline only or of caffeine dissolved in saline (5 mg caffeine/kg body weight) were administered to the no caffeine and caffeine groups, respectively. After 9 wk of training, body weight and epididymal and retroperitoneal fat-pad weights were significantly reduced in the caffeine groups and in the exercise groups (P less than 0.05). Epididymal fat-cell size was significantly reduced by caffeine treatment, and exercise training reduced fat-cell diameters in both epididymal and retroperitoneal fat-pads (P less than 0.05). The additional 22% reduction in body weight, 25% reduction in epididymal fat-cell size, and 5 and 6% reductions in epididymal and retroperitoneal fat-pad weights, respectively, in the caf-ex group beyond the no caf-ex group support the hypothesis that fat loss with aerobic exercise can be increased when caffeine is ingested prior to the training sessions.