Progesterone (P) stimulates estradiol dehydrogenase (E2DH) activity in human proliferative endometrium. The potencies of several progestins were studied in this system. Endometrium was incubated for 2-3 days in a culture medium containing various concentrations of steroids. The E2DH activity was measured at the end of the incubation. Steroid specificity was studied by examining the dose-response characteristics for each progestin in the stimulation of the enzyme activity. In this system, the most potent progestins, medroxyprogesterone acetate, R-5020 and P, were able to stimulate endometrial E2DH at a concentration as low as 3 nM after 2 days of incubation. 17.alpha.-Hydroxyprogesterone (17-OHP) caproate was effective at 30 nM, whereas high concentrations (> 100 nM) of medroxyprogesterone and 17-OHP were required to produce a detectable increase in activity. All of these compounds, except 17-OHP, produced a dose-dependent increase in E2DH activity. Based on the relative stimulation observed, it appears that a hydroxyl group, especially at the 17.alpha. position, greatly reduces the ability of a progestin to stimulate the enzyme. However, this capacity is restored by esterification of the hydroxyl group. Corticosterone (B), but not cortisol, was found to be able to increase E2DH activity. However, the dose-response curve was entirely different from that of the progestins. At least 200 nM B was required before an increase in enzyme activity could be detected. Kinetic analysis of the stimulation of E2DH revealed that the maximum enzyme activity stimulated was similar when using either P or B. However, the concentrations required to reach the half-maximum enzyme activity were very different (10 nM for P and 1400 nM for B). B may not be physiologically significant as a stimulator.