The effect of prolonged physical and psychological stress on the testicular function was studied in 8 students (age 22–25 years) of the Norwegian Academy of War during a combat course of 5 days' duration. The average urinary excretion of free cortisol and 17‐ketogenic steroids was 81 and 129% higher than the respective control values one week after the course. Plasma cortisol levels increased from 21.7 μg/100 ml at 8 a. m. before the course to 24.6 (P < 0.05), and serum HGH rose from undetectable levels, < 0.08 ng/ml, to an average value of 12.9 ng/ml ± 3.7 (SD) at 8 a. m. during the course. A marked suppressive effect on plasma testosterone levels from 5.6 ng/ml ± 1.4 to 0.9 ± 0.5, and no adjustment to stress was observed over a 5 day period. TeBG increased gradually from 26.9 nmol/l ± 9.9 to 52.7 ± 17.7 on day 6, followed by a slow decrease without reaching control values on day 12, suggesting that the decreased plasma testosterone levels probably reflect reduced production and not increased metabolism of testosterone. LH fluctuated during the course, but was significantly higher in the morning immediately following the end of the course than at the start (P < 0.02). It is postulated that the effect of stress on the plasma testosterone levels is mediated via an action both on the hypothalamus‐pituitary level and on the testis.