The changes in oestradiol, testosterone and prolactin, and the effects of a high calorie diet, were investigated in eleven young males cadets participating in a 5 days' ranger training course involving heavy and continuous physical activities with less than 1500 kcal . day-1 and almost without sleep. Prolactin decreased continuously during the first 24 h to 25% of precourse values. Testosterone decreased during the day time and reached below 25% of precourse values after 48 h. Oestradiol did not change during the first 48 h but then decreased to about 50% of precourse values. In conclusion, oestradiol, testosterone and prolactin are strongly reduced during prolonged strain. These changes were not reversed by a high calorie diet.