The effects of strenuous physical exercise were studied on the pituitary-testicular response to gonadotropin-releasing hormone (GnRH) stimulation and on growth hormone (GH) and cortisol secretion. Eight healthy adult males were injected twice intravenously with 0.1 mg of GnRH at intervals of 21 days. At the time of the first injection (exercise trial) the subjects had been bicycling for 4 h on the road, and at the time of the second injection (rest trial) they had been resting in a sitting position for 4 h. Blood samples were taken before and after the 4-h period and 30, 60 and 120 min after the GnRH injection. Both testosterone and luteinizing hormone (LH) decreased during the exercise by 18% (p = 0.037) and 29% (p = 0.0028), respectively, but increased after the GnRH injection by 1.4- and 12.9-fold (p = 0.0001 for both). The areas under the testosterone and LH response curves after GnRH were significantly larger in the exercise trial than in the rest trial, threefold (p = 0.013) and 1.3-fold (p = 0.0007), respectively. Growth hormone and cortisol increased during the exercise trial. In the rest trial, the GnRH injection increased serum GH concentrations (p = 0.027). In conclusion, the diminished hypothalamic GnRH secretion seems to be the major cause of the post-exercise decrease in LH and testosterone, but altered sensitivity of the pituitary to GnRH also may be involved. Apparently, the build-up of pituitary LH stores during exercise explains the enhanced LH and testosterone response to GnRH challenge after exercise in comparison to control GnRH challenge.