Abstract The addition of diethylstilbestrol to cultured rabbit endometrial cells increased the number of DNA-replicating cells; progesterone had an opposite effect. However, both hormones induced a significant increase in the incorporation of labeled uridine and aminoacids into TCA precipitates. These hormonal effects were rapidly reversed upon removal of the hormones from the culture medium and were also inhibited by actinomycin D and cycloheximide. Hydrocortisone did not change the effect produced by diethylstilbestrol on DNA replication but inhibited the progesterone action. Dibutyril 3',5'-cyclic AMP did not alter the effect of the same hormones on DNA replication. However it was surprisingly found to inhibit the hormonal effects on the incorporation of labeled uridine and amino acids. Neither sex hormone altered the specific activity of alkaline, acid phosphatase, or lactic dehydrogenase, or the uptake of 3- O-methyl- d-glucose or α-aminoisobutyric acid. It is suggested on the basis of our findings, (a) that progesterone induces these endometrial epithelial cells to differentiate in vitro into non-dividing, secretory cells and (b) that there is no need for hyperemic and/or cell membrane transport changes for the expression of these estrogenic effects.