To determine whether short periods of fasting can suppress the activity of the reproductive axis in normal healthy men, eight men were studied on a fed day and again after 48 h of fasting. Subjects were between 20–32 yr of age and ranged from 84–119% of normal body weight. Blood samples were collected on day 1 (a fed day) and day 3 (after 48 h of fasting) at 15-min intervals from 0800–1600 h through indwelling venous catheters. Fasting for 48 h resulted in a significant decrease in mean LH, FSH, and testosterone concentrations. The mean LH concentration decreased from 2.94 ± 0.59 IU/L on the fed day to 1.07 ± 0.14 IU/L after 48 h of fasting, and there was an accompanying decrease in LH pulse frequency (from 5.13 ± 0.29 to 2.63 ± 0.62 pulses/8 h) and mean baseline LH concentration (from 1.83 ± 0.52 to 0.51 ± 0.07 IU/L), but no significant decrease in LH pulse amplitude. In a second study, blood samples were collected from five subjects who were allowed to eat normally between days 1 and 3; these individuals showed no difference in LH secretion. To begin to examine the possibility that an activation of the hypothalamic-pituitary-adrenal axis leads to the suppression of reproductive hormone levels that occurred after 48 h of fasting, cortisol levels were measured in all plasma samples. There was no significant difference in mean cortisol concentrations on fed us. fasted days or when cortisol concentrations were examined as hourly means across the 2 days. These results indicate that activity of the reproductive axis can be suppressed in normal healthy men by 48 h of fasting. It appears unlikely that activation of the adrenal axis is the cause of this suppression of reproductive axis activity.