The ability of progesterone to modulate the production and secretion of human CG (hCG) in both normal placenta and choriocarcinoma was compared by culturing explants of each trophoblastic tissue in the presence or absence of progesterone. The cellular level of messenger RNAs (mRNAs) encoding hCG alpha, hCG beta, and human placental lactogen (hPL) were quantitatively estimated by mean grain count per syncytial nucleus on the tissue sections hybridized in situ with labeled complementary DNA probes corresponding to these mRNAs. Immunoreactive hCG, hCG alpha, and hCG beta in the media and explanted tissues were measured by the homologous RIAs, and hPL was assayed by hPL-RIA kit. Addition of progesterone at concentrations of 5-20 micrograms/ml into the culture of normal early placenta caused a decrease in the cellular levels of hCG alpha mRNA and hCG beta mRNA after a 24-h culture, and exhibited a decline in immunoreactive hCG and hCG alpha levels released into the media together with a decrease in immunoreactive hCG alpha and hCG beta levels in the explanted tissues after a 48-h culture. The addition of progesterone neither affected the cellular levels of hPL mRNA nor immunoreactive hPL levels in the media and tissues. On the other hand, addition of 17 beta-estradiol at concentrations similar to those used with progesterone did not alter the levels of immunoreactive hCG and hCG alpha in the media or explanted placental tissues, while lower concentrations (1-10 ng/ml) of 17 beta-estradiol caused an increase in immunoreactive hCG alpha levels in the media and cultured tissues. These findings suggest that the suppressive effect observed with progesterone is not likely to be a toxic effect of steroid, but is rather selective on hCG production and secretion by normal placenta. Thus, progesterone may be a factor responsible for the inhibitory regulation of hCG production and secretion by normal placenta. However, in contrast to normal placenta, the choriocarcinoma culture in vitro did not respond to progesterone. The steroid was without significant effect on the cellular levels of hCG alpha mRNA and hCG beta mRNA, and on the levels of immunoreactive hCG and hCG alpha in the media and explanted tissues. These results suggest that the inhibitory regulation of hCG production and secretion in choriocarcinoma is different from that in normal early placenta.