The distribution of melatonin receptors in six discrete brain areas of mature (3–4 months old) and aged (>24 months old) male rats was recorded every 4 h during a 24-hour light: dark cycle (L:D 14:10 h). I25I-melatonin was used as a melatonin receptor probe. In the mature animals, specific binding of 125I-melatonin was found in all brain areas investigated, i.e. hypothalamus, medulla pons, hippocampus, cerebellum, parietal cortex and striatum. The density of 125I-melatonin-binding sites in the hypothalamus, medulla pons and hippocampus exhibited clear diurnal rhythms with different patterns and phases. No such rhythm was evident in the cerebellum, parietal cortex and striatum. The apparent affinity of the binding sites was similar in all the brain regions and did not change at any of the times recorded. In the old male rats, the density of 125I-melatonin binding sites in the hypothalamus was only 10% of that in the mature animals at 13 h after the onset of light and was vanishingly small throughout the 24-hour period. The 24-hour mean of the binding site density in the parietal cortex, hippocampus and medulla pons was significantly lower than in mature rats with no apparent diurnal variations. The age-related decrease in the density of melatonin-binding sites was less pronounced in the cerebellum and striatum. In all brain areas tested, apart from the hypothalamus, the decrease in receptor densities was not accompanied by changes in the apparent affinity towards the ligand. Serum concentrations of melatonin measured by radioimmunoassay exhibited a significant nocturnal rise in the rats, but in the aged this rise was about one third of that in the mature animals. The results clearly indicate diurnal rhythms in 125I-melatonin binding in the hypothalamus, medulla pons and hippocampus of mature rats, vanishing in the aged animals despite persistence of rhythm in circulating melatonin. In addition, a definite age-related decline in melatonin receptors is evident, particularly in the hypothalamus and hippocampu