In two experiments, food was withheld from male rats beginning at 12:00 p.m. Fasted rats and fed controls were killed 8, 16 and 24 hours later. Rats were 4 and 6 months old in experiments 1 and 2, respectively. Fasted rats had lower (P<0.01) serum levels of LH, FSH and glucose than did fed rats; serum levels were independent of the duration of the fast. Serum levels of testosterone were not significantly affected by fasting. Serum levels of glucose and testosterone varied with time of day (hour, P<0.01) and were highest at 12:00 p.m. and 4:00 a.m. respectively. Fasting had a greater effect on serum glucose levels in experiment 1 than in experiment 2 (experiment × fasting, P<0.01), and the pattern of change in FSH levels over time of day differed in the two experiments (experiment × hour, P<0.01). 1 This study was supported by a grant (MA-4454) from the Medical Research Council of Canada