Secretory profiles of prolactin and growth hormone (rGH) over a period of 6 h were obtained from unrestrained pinealectomized or sham-operated control male rats, both groups having been chronically implanted with right atrial cannulae and acclimatized to isolation in special blood sampling cages with a strict cycle of 12 h light : 12 h darkness (L : D). Hormone profiles were obtained from 2 h before to 4 h after the commencement of the light period. In intact animals, secretion of prolactin was episodic without a clear ultradian rhythm except that secretory bursts tended to be larger during the dark period. Furthermore, a peak of prolactin occurred regularly at the commencement of the light period, even though subsequent peaks were diminished in amplitude. Mean concentrations of prolactin during the dark period were significantly higher than later daytime values, supporting the view that there is a diurnal prolactin rhythm. Pinealectomy altered the secretory patterns in prolactin so that peaks became more prominent early in the light period but mean concentrations of prolactin eventually fell as they did in control animals and there was no difference in concentrations of prolactin over a period of 6 h between the two groups. Pinealectomy, therefore, slightly delayed the fall in mean prolactin concentrations which normally occurs in the light period. Secretion of rGH was rhythmic with interpeak intervals and mean concentrations in which there were no differences between control and pinealectomized groups. Inhibition of secretory episodes of rGH by the interface between darkness and light was impaired in pinealectomized animals with a delay in the appearance of the first diurnal trough. It was concluded that pinealectomy attenuates L : D cycle regulation of secretory rhythms of prolactin and rGH to a minor degree.