Acute doses of Sinemet® (L-DOPA combined with carbidopa) previously failed to change prolonged exercise performance in a temperate environment, but it is not known whether acute doses of L-DOPA timed to reach Cmax during exercise will improve prolonged cycling performance in warm conditions. 10 physically active men (age 26 ± 4 y; height 1.76 ± 0.08 m; body mass 76.3 ± 10.6 kg; VO2peak 57 ± 8 ml/kg/min) were recruited for this study. Participants cycled for 1 h at 60% VO2peak followed by a 30 min exercise test, during which they were instructed to complete as much work as possible. Heart rate, skin and core temperature, as well as ratings of perceived exertion and thermal comfort were recorded throughout exercise and blood samples were collected at rest, 15 min intervals during the first hour of exercise and the end of the exercise test. Finger tap tests at the beginning and end of exercise were employed to examine fine motor control. There was no significant difference in the work done on placebo (314 ± 43 kJ) and L-DOPA trials (326 ± 48 kJ; p = 0.08). Prolactin concentrations were increased at the end of exercise in all trials (p < 0.001) but this response was attenuated at the end of exercise for the L-DOPA trial (11.4 ± 5.5 ng/ml) compared to single-blind (23.6 ± 5.6 ng/ml) and double-blind placebo trials (20.8 ± 3.3 ng/ml; p = 0.024). No differences between trials were found for any other measure. The results suggest that increasing central catecholamineavailability inhibits the normal prolactin response to exercise in the heat, but does not alter performance, thermoregulation or sympathetic outflow.