After the administration of dehydroepiandrosterone sulfate (DHAS) (0,10,30,70 mg/kg, i.v.) to rabbits during late pregnancy, serum concentration of DHAS, dehydroepiandrosterone (DHA), testosterone (T), estradiol (E2) and progesterone (Prog) were determined by radioimmunoassay. Serum levels of DHAS increased dose-dependently, and were reduced biphasically to normal levels at 24 hr after the administration. Similarly, the levels of DHA, T and E2 increased, and reached the maximum at 30 min after, though the increase of DHA or T was one-thousandth that of DHAS, and that of E2 was about one-hundred-thousandth. On the other hand, Prog concentration in sera decreased to 50 approximately 60% of that before the injection at 30 min after (p less than 0.05). These results suggest that the changes observed in E2 and Prog levels after the injection of DHAS into pregnant rabbits corresponded to the changes of endogeneous E2 and Prog seen in early parturition in women.