Inhibition of aromatase, a key enzyme in the biosynthesis of oestradiol-17β, by the addition of 1,4,6-androstatrien-3,17-dione resulted in a significant increase in the levels of immunoreactive human chorionic gonadotrophin (hCG) in the medium and tissue. This increase was partially reversed by the simultaneous addition of oestradiol-17β. These effects on the levels of immunoreactive hCG were also reflected by the increased levels of mRNA specific for the α and β subunits of hCG following the addition of the aromatase inhibitor. However, addition of tamoxifen resulted in a drastic decrease in the levels of both the messages. Based on these results, it is suggested that the synthesis of hCG is negatively modulated by oestradiol-17β in the human placenta.