After validation of the methodology, melatonin receptors have been measured by binding of (125I)-melatonin to membranes of individual pars tuberalis/median eminence of lambs at different times of the day in the course of three experiments. Plasma melatonin was assessed by radioimmunoassay 60 min, 30 min and just before slaughter. Kd was found in the range of 10-25 pM and did not vary with the time of slaughter. Particularly, the mean Kd values were identical and equal to 17 pM in animals slaughtered either during the light or the dark phase of the day. The pattern of Bmax changes was similar in the three experiments and varied significantly (p < 0.001) with the time of the day. The apparent numbers of receptors were found to be the highest at the end of the day and at the onset of the night and to be the lowest at the end of the night: 63.4 +/- 6.3, 61.4 +/- 6.5 and 34.4 +/- 2.8 fmol/mg protein (mean +/- SEM), respectively. Values were found to be intermediary either at the middle of the day (51.1 +/- 5.3 fmol/mg protein) or at the middle of the night (40.4 +/- 6.3 fmol/mg protein). Furthermore, plasma melatonin was positively correlated to the melatonin receptor number (p < 0.02) but only at the onset of the night. In conclusion, the results are strongly suggestive of the existence of a circadian rhythm in the apparent number of melatonin receptors in the lamb pars tuberalis/median eminence with a possible downregulation occurring during the night.