Indoleamine levels in punches of the medial hypothalamus containing the suprachiasmatic nuclei (SCN) of 4-week-old cockerels were determined by HPLC-EC. Melatonin levels in punches were determined by radioimmunoassay (RIA). Daily rhythms of serotonin (5-HT) and of its metabolite 5-hydroxy-3-indoleacetic acid (5-HIAA) were observed; levels were higher at midnight than at midday. A daily rhythm with the same phase in punch melatonin content was also observed. Pinealectomy at 1 week after hatching abolished the 5-HIAA and melatonin rhythm in 4-week-old birds but did not abolish the 5-HT rhythm. Injections of melatonin (0.5 mg/kg) increased 5-HT, 5-HIAA and melatonin levels in the hypothalamic punches. These results indicate that circulating melatonin of pineal origin may act to increase 5-HT turnover and/or release in the SCN. They suggest a link between the circadian secretion of pineal melatonin and the regulation of 5-HT projections to the hypothalamus from the raphe nuclei in the brainstem of the chicken. We have previously shown that the rhythmic secretion of melatonin by the pineal is influenced by oscillators in the brain via the superior cervical ganglia. The results reported here indicate that melatonin in turn may regulate brain oscillators, suggesting a neuroendocrine loop within the avian circadian system.