The effect of several years' endurance training on hormonal changes during acute prolonged physical exercise was studied. In trial I, 13 cross-country skiers were studied before and after a 75-km ski race and 3 weeks later on a control day. In trial II, 10 trained and 8 untrained subjects bicycled for 4 h on the road with as high a performance level as possible. Venous blood samples were taken in both trials before and after the exercise. In trial I, serum concentrations of luteinizing hormone (p < 0.01) and follicle-stimulating hormone (FSH, p < 0.001) decreased more from morning to afternoon samples, and cortisol (p < 0.001) and growth hormone (GH, p < 0.001) increased more during the ski race when compared to the control day. Serum testosterone decreased during the ski race (p < 0.01) but not on the control day. In trial II, a group difference was seen in serum FSH levels which was higher in the trained than untrained subjects at all three time points (F = 4.66, p = 0.046). A significant trial-group interaction (p = 0.020) was seen in GH, the GH being lower before exercise and higher 2 h after exercise in the untrained subjects. There was a significant group contrast between pre- and post-exercise samples in testosterone (p = 0.021) and cortisol (p = 0.022). In conclusion, the higher basal FSH concentration in the trained versus untrained subjects may be a sign of compensated hypogonadism due to intensive chronic training or it may be due to dysfunction of Sertoli cells.(ABSTRACT TRUNCATED AT 250 WORDS)