Sports influence the hypothalamus-pituitary-gonadal axis (HPG). The purpose of this study was to analyze the effects of physical stress on qualitative male LH secretion. We evaluated the HPG axis at rest and the LH response to a treadmill exercise (90 % of Vdot;O 2 max) in eight trained (running 50 to 70 km/week) and eight untrained subjects. FSH, total testosterone (T) and free testosterone (FT) were evaluated at rest by radioimmunoassay, whereas serum LH was evaluated both by radioimmunoassay (I-LH) and by a biological assay (RICT, B-LH), before (-30, -15, 0 pre) and after exercise (0 post, + 15, + 30, + 60 min). Semen analysis was performed. The results showed that the B/I LH ratio was significantly lower at rest in athletes than in controls (1.3 +/- 0.5 and 2.1 +/- 0.7 respectively, p < 0.05) and the B-LH and B/I LH ratio were reduced after exercise in controls (from 21.1 +/- 5.7 to 13.9 +/- 3.4 mIU/mL and from 2.3 +/- 0.8 to 1.6 +/- 0.4, at + 60 respectively, p < 0.05). T and FT were significantly lower in athletes compared with controls (4.18 +/- 1.1 vs 5.96 +/- 1.6 ng/ml and 24.3 +/- 4.8 vs 29.7 +/- 5.2 pg/ml respectively, p < 0.05). Semen parameters showed no difference. The B/I LH ratio could be a "marker" of altered LHRH secretion due to an exercise-linked adaptive condition.