The effect of sleep on the serum levels of hormones during prolonged heavy physical strain and calorie deficiency were investigated in 19 young men participating in a 5 day ranger training course with a calorie consumption of 35,000-50,000 kJ X 24 h-1, and a calorie intake of about 6,000 kJ X 24 h-1. The subjects were divided into two groups: the stress group (8 cadets) were allowed no organized sleep during the course, whereas the sleep group (9 cadets) had 3 h sleep each night. Small but significantly (p less than 0.01) higher serum levels were found in the sleep group compared to the stress group for cortisol, growth hormone and testosterone. No such differences were found for catecholamines, androstendione, dihydrotestosterone, LH, triiodothyronine and thyroxine. Androstendione and dihydrotestosterone decreased in parallel with testosterone (r = 0.5) during the course, changes which directly or indirectly seem to be due to decreased testicular secretion. The changes found during this investigation for the other hormones are similar to changes found during previous courses. LH showed only small variation during the course and cannot explain the decreased secretion from the testis or the difference between the two groups for testosterone. All hormones were normal within 23 days after the end of the course.