The glycogen content and glycogen synthetase and glycogen phosphorylase levels were studied in endometrial samples obtained from 19 normal and 37 infertile patients during the menstrual cycle before and after administration of progestogen. Each of the above groups received the progestogen Lyndiol (lynestrenol, 5.0 mg, and mestranol, 0.15 mg) daily for 7 days during the follicular phase of the menstrual cycle. In both groups an increase in the endometrial glycogen deposition and an increase in glycogen synthetase enzyme levels were seen. When the administration of progestogen was started on day 7 after ovulation, during the luteal phase, the glycogen content of endometrial tissue from infertile patients increased significantly; no change was found in endometrial samples from normal patients. No difference was found in the serum progesterone levels of normal and infertile patients in the midsecretory phase of menstrual cycle, and Lyndiol reduced the serum level of progesterone to approximately that found during the follicular phase in untreated normal women. These studies suggest that the proliferative endometrium of infertile patients may be less stimulated by ovarian estrogen than is normal endometrium, whereas endometrial tissue obtained from both groups during the luteal phase responded similarly to progesterone in glycogen synthesis and storage.