['The effects of GH on exercise performance remain unclear.', 'The aim of the study was to examine the effects of GH receptor (GHR) antagonist treatment on exercise performance.', 'Subjects were treated with the GHR antagonist pegvisomant or placebo for 16 d. After the treatment period, they exercised to determine exercise performance and hormonal and metabolic responses.', 'Twenty healthy males participated in the study.', 'Subjects were treated with the GHR antagonist (n = 10; 10 mg/d) or placebo (n = 10). After the treatment period, they performed a maximal oxygen uptake (VO(2 max)) test and a prolonged exercise test, consisting of 60 min of submaximal cycling followed by exercise to fatigue at 90% of VO(2 max).', 'VO(2 max) was measured before and after the treatment period. Hormonal and metabolic responses and time to exhaustion during prolonged exercise were determined.', 'Resting serum IGF-I concentration decreased by 20% in the GHR antagonist-treated group (P < 0.05), whereas no change was observed in the placebo group. Conversely, resting serum GH concentration was significantly higher in the treatment group compared with the placebo group (P < 0.01). VO(2 max) did not change significantly in either group after the treatment period. Time to exhaustion at 90% of VO(2 max) was significantly shorter in the treatment group (P < 0.05). No significant differences were observed between the groups in terms of changes in serum free fatty acids, glycerol, VO(2), or relative fat oxidation.'] Does growth hormone receptor antagonist treatment reduce exercise performance in young males?