To determine whether adrenocortical function influences rhythmic gonadotropin patterns in male rats, blood samples were obtained by rapid decapitation from control and adrenalectomized male rats at 3-h intervals over a 24-h period (14 h light/day, 4 a.m.–6 p.m.). Control animals showed periodicity in serum LH, prolactin (Prl) and corticosterone similar to that reported previously. Peak and trough serum levels of corticosterone and Prl occurred at 5 p.m. and 8 a.m. respectively whereas those for LH occurred at 11 p.m. and 11 a.m. Serum FSH levels did not vary with circadian periodicity. 15 days after adrenalectomy, serum LH levels continued to fluctuate with a circadian periodicity but showed a marked phase shift; peak LH levels were observed at 8 a.m. The absence of adrenal hormones did not result in altered FSH levels nor was the phase of the Prl rhythm significantly altere