Nine healthy endurance-trained males were recruited to examine the effect of a dual dopamine/noradrenaline reuptake inhibitor on performance, thermoregulation and the hormonal responses to exercise. Subjects performed four trials, ingesting either a placebo (pla) or 2 x 300 mg bupropion (bup), prior to exercise in temperate (18 degrees C) or warm (30 degrees C) conditions. Trials consisted of 60 min cycle exercise at 55% W(max) immediately followed by a time trial (TT). TT performance in the heat was significantly improved by bupropion (pla: 39.8 +/- 3.9 min, bup: 36.4 +/- 5.7 min; P = 0.046), but no difference between treatments was apparent in temperate conditions (pla: 30.6 +/- 2.2 min, bup: 30.6 +/- 1.9 min; P = 0.954). While TT power output was consistently lower in the heat when compared to temperate conditions, this decrement was attenuated by bupropion. At the end of the TT in the heat, both core temperature (pla 39.7 +/- 0.3 degrees C, bup 40.0 +/- 0.3 degrees C; P = 0.017) and HR (pla 178 +/- 7 beats min(-1), bup 183 +/- 12 beats min(-1); P = 0.039), were higher in the bupropion trial than in the placebo. Circulating pituitary and adrenal hormone concentrations increased throughout exercise in all trials. Circulating serum prolactin was elevated above temperate levels during exercise in a warm environment (P < 0.001). These data indicate that performance in warm conditions is enhanced by acute administration of a dual dopamine/noradrenaline reuptake inhibitor. No such effect was apparent under temperate conditions. It appears that bupropion enabled subjects to maintain a greater TT power output in the heat with the same perception of effort and thermal stress reported during the placebo trial, despite the attainment of a higher core temperature.