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DAILY variations in melatonin content and 2-[125I]melatonin specific binding in retinas of golden hamsters were studied. Both parameters showed significant variations throughout the 24 h period. Maximal specific binding was observed at 24.00 h, while melatonin content peaked at 04.00 h. Saturation studies performed at 12.00 and 24.00 h indicated that the maximal concentration of 2-[125I]melatonin binding sites (Bmax) was significantly higher at 24.00 h than at 12.00 h, whereas the dissociation constant (Kd) remained unchanged. As 2-[125I]melatonin specific binding decreased at times when retinal melatonin content was high, these findings suggest that daily variations in retinal melatonin levels may be implicated in the regulation of the density of melatonin binding sites, probably through mechanisms of up-and down-regulation induced by melatonin on its own binding sites.