A radioautographic analysis of the number and distribution of labeled nuclei after in vitro incorporation of radiothymidine was carried out in human endometrium during the normal menstrual cycle. The highest rates of proliferation were recorded on cycle days 8 to 10 in the upper one third of the functionalis layer. Coinciding with intraglandular secretory activity on cycle day 19, the rate of proliferation of gland cells decreased near zero levels and remained unchanged thereafter. Midsecretory phase arteriolar coiling and stromal predecidualization were associated with increased isotope incorporation. DNA synthesis in premenstrual endometrium was limited to the surface epithelium, predecidua, and vascular endothelium. The endometrium of the isthmus and cornual regions and of the basalis zone demonstrated relatively constant and comparatively lower rates of proliferation throughout the cycle. Regional and cellular variations in nucleic acid synthesis in cyclic endometrium are likely to reflect differences in physiologic functions in target cells in response to cyclic hormonal stimuli.