This study investigated the effects of a heavy resistance exercise session (RES) with the oral daytime ingestion of melatonin on the physiological responses and acute performance. In a randomized and double-blind controlled study 10 healthy male subjects undertook an 80 min intensive hypertrophic RES for major muscles of the lower and upper extremities. The subjects were studied on two occasions receiving either melatonin (6 mg) or placebo (6 mg) in random order 60 min before each RES. Blood samples were taken from an antecubital vein both in fasting conditions in the morning and before RES (pre 60 min, pre 0 min), during RES (middle) and after RES (post 0 min, post 15 min, post 30 min, post 60 min). Maximal jumping ability and maximal strength in bench press and squat were measured before and immediately after RES in which there were no differences between the melatonin and placebo groups. The serum melatonin concentration increased significantly (P<0.05-0.001) in the melatonin group following oral ingestion of melatonin and was elevated at every time point after that. The concentration reached a peak value of 1171.3+/-235.2 pg/ml in 60 min at pre 0. Serum melatonin increased slightly but significantly (P<0.05) also in the placebo group just before RES, in the middle of RES and after RES (post 0, post 15). There were large differences (P<0.01-0.001) in the serum melatonin concentration between the groups at all time points. There were no differences in the growth hormone (GH), testosterone and cortisol peak concentrations at any time points between the groups but the area under the curve of GH during RES (P<0.01) and during the 60 min after RES (P<0.05) was lower in the melatonin condition. In conclusion, the present findings give evidence that oral ingestion of melatonin (6 mg) during daytime with heavy resistance exercise may slightly decrease GH concentrations. On the other hand, it seems that melatonin administration during daytime does not have any acute (1-2 h) effects either on the maximal jumping ability or on the maximal strength.