An incubation procedure for the demonstration of melatonin is described using freeze-dried, unfixed material embedded in vacuum directly in glycolmethacrylate. The results indicate the binding of melatonin in the nucleus of pineal parenchymal cells and in the outer nuclear layer of the retina of rats; binding is greater at night than during the day.

These results provide insight into the regulation of the autocrine and paracrine factor EGF in the endometrium during the preimplantation period.