Melatonin in the retina of rats: a diurnal rhythm.

Melatonin was extracted by chloroform from the retina of the rat and measured by radioimmunoassay. In rats housed under a régime of 12 h light: 12 h darkness, melatonin content in the retina, like that in the pineal, serum and brain tissue, was high from midway through the dark to early in the period of light, and low from half-way through the light period to early in the period of darkness. A similar result was found in a second experiment. The existence of a diurnal rhythm of melatonin in the retina of rats as observed in this study is consistent with the suggestion that melatonin may regulate the diurnal rhythm of eye pigmentation in vertebrates.