Ovaries from 8-week-old female NMRI mice, treated with 5 micrograms diethylstilbestrol (DES) daily for the first 5 days after birth were studied for progesterone synthesis in vitro, using 3H-pregnenolone as precursor, and assayed for progesterone content using radioimmunoassay (RIA) technique. The plasma levels of progesterone were measured with RIA. These results were compared with those from control ovaries from females in different phases of the estrous cycle. Values for radioactivity representing 3H-progesterone were significantly higher after 1-hour incubation of ovarian homogenate from DES treated females than in homogenates of ovaries from any phase of the normal estrous cycle. The ovarian content of progesterone was similar in DES exposed ovaries and ovaries from females in proestrous or estrous but lower in DES ovaries than in diestrous ovaries. The plasma levels of progesterone were in the same range in DES treated females and diestrous females but higher in DES females than in estrous or proestrous females.