Alkaline phosphatase was considered as evidence of estrogen-induced protein synthesis in the endometrium. Biochemical and histochemical determinations of alkaline phosphatase activity have been made on human endometrium during the whole cycle, with simultaneous histologic appraisal. Fifty-four cases were used to compute a curve of alkaline phosphatase activity during the menstrual cycle, the peak of which appears to be on the eighteenth day. Although the data are preliminary and the observations few, the results suggest that such a method would be accurate to estimate estrogen action upon the endometrium.