A single dose of oestradiol stimulates proliferative activity in the endometrium of castrated rats. This was tested autoradiographically by measuring 3H-thymidine and 3H-uridine incorporation and by determining the mitotic index. Administration of gestagens simultaneously or at different time intervals after oestradiol treatment (early G1 or early S phase) resulted in an inhibition of 3H-thymidine uptake which was greatest with simultaneous application of the two steroids. With regard to 3H-uridine incorporation into the different uterine cell types, progesterone produced a significant inhibition of RNA synthesis in all three cell types most pronounced in the myometrium. The role of these effects is discussed with regard to the growth-inhibiting effect of gestagens in human endometrial carcinoma.