SUMMARY The activity of several enzymes has been measured in the uterine endometrium of the rabbit during oestrus and pseudopregnancy and after injecting oestradiol benzoate or progesterone 28 days after ovariectomy. The enzyme activity of the uterine fluid has been determined during oestrus and the effect of uterine ligation studied. Progesterone and the induction of pseudopregnancy stimulated succinic dehydrogenase (SDH) and glucose-6-phosphate dehydrogenase (GDH) activity and depressed amylase and lactic dehydrogenase (LDH) activity. In ovariectomized does, glutamate-oxaloacetate transaminase (GOT) activity increased after the injection of progesterone. Progesterone also stimulated endometrial phosphatase after ovariectomy but, when given after a period of oestrogen treatment, it limited the even greater response of acid and alkaline phosphatase to oestrogen; the activity then attaining the same level as when progesterone alone was given. SDH, GDH and glycerylphosphorylcholine (GPC) diesterase could not be detected in uterine fluid but amylase and alkaline phosphatase were in greater concentration than in the endometrium. GPC diesterase was, however, found to be present in uterine tissue. Ligation of the uterus did not significantly alter the enzyme activity of the endometrium.