States of chronic undernutrition can cause a profound suppression of reproductive function. To begin to determine the time course and the nature of the mechanism by which undernutrition suppresses the activity of the reproductive axis we have examined the effects of brief periods of fasting on reproductive hormone secretion in men and male rhesus monkeys. In monkeys there is a significant suppression of pulsatile luteinizing hormone (LH) and testosterone secretion after a single day of fasting, that is apparent within the first 4 h after a meal is missed. The suppression of pulsatile LH secretion on a day of fasting does not appear to be caused by the psychological stress experienced when monkeys are deprived of their daily meal in that monkeys who are maintained in a metabolically fed state (by feeding a large excess of food on the day prior to fasting), but are deprived of a meal and displayed behavioural agitation associated with fasting, have no suppression of LH secretion. The suppression of LH secretion on a day of fasting cannot be reversed by naloxone infusion, indicating that increased secretion of opioid peptides is not the primary mechanism causing the decrease in the central drive to the reproductive axis during fasting.(ABSTRACT TRUNCATED AT 250 WORDS)