Day-night differences in the ultrastructure of pineal neuronal elements were quantitated in the rat and the chick – two species that exhibit important differences in adrenergic control of melatonin synthesis. Area densities of small clear vesicles (40–60 nm) and small dense-cored vesicles (40–60 nm) in nerve terminals were significantly reduced during the night in the rat. In the chick, there were no day-night differences in the frequency of clear vesicles, but the number of small dense-cored vesicles tended to be greater at night. There was no effect of day-night sampling on area densities of large dense-cored vesicles (80–120 nm) in either species. These findings are consistent with biochemical evidence for day-night species differences in the adrenergic control of pineal f
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