Nonstress blood samples were obtained from intact and thyroidectomized (TE) male rats at 3-hr intervals over a 24-hr period via rapid decapitation. The animals were thyroidectomized when 40 days old and used 6 weeks later. Intact animals showed periodicity in serum LH (P < 0.01) and prolactin (P < 0.01). Both gonadotropins began increasing after 8 pm and peak levels occurred at 11 pm. In contrast, 24-hr periodicity was not observed in serum FSH. Corticosterone levels in these same serum samples showed the expected circadian periodicity. After TE, the 24-hr pattern in all gonadotropins was altered significantly. Serum LH increased (P < 0.01) and circadian periodicity appeared to be absent. FSH and prolactin levels were increased and decreased, respectively (P < 0.01), with serum prolactin showing a 9-hr phase shift. Prolactin began increasing at 2 am and reached a peak at 8 am. Corticosterone in TE animals showed a 24-hr rhythm similar to that of intact rats. These findings confirm our previous observations that non-stress serum LH and prolactin levels fluctuate with a 24-hr periodicity and suggest that the level of, and the phase angle between, these rhythms is markedly influenced by pituitary-thyroid activity.