During a five days' military training course for male cadets with hard physical activity day and night and almost no sleep or food, a decrease was found in LH, FSH, PRL and TSH. A decrease was also found in testosterone, dihydrotestosterone (DHT), androstenedione, dehydroepiandrosterone and 17 alpha-OH progesterone, whereas dehydroepiandrosterone-sulfate increased twofold. The LH and FSH responses to GnRH intravenously were increased at the end of the course. This demonstrates enhanced pituitary reserves of gonadotropin, or, alternatively, increased sensitivity to GnRH stimulation and may be due to decreased hypothalamic secretion of GnRH during the course. The decreased DHT and testosterone levels were almost normalized after HCG stimulation, indicating a gonadotropin regulated decrease in testosterone secretion during the course. In spite of fairly weak correlation between the alteration in gonadotropins and androgens it is concluded that there is a major regulation of testicular androgen secretion during prolonged stress by the hypothalamo-pituitary axis.