The present study investigated possible sites through which ACTH or corticosterone inhibit progesterone secretion in pregnant rats, and the role of placental factors in blocking the inhibitory effect. The number of conceptuses was adjusted to one (1C group) or more than ten (FC group) on day 7 of pregnancy by aspirating the desired number. Serum concentrations of progesterone, testosterone and oestradiol were significantly (P<0·01) lower on day 15 in the 1C group than in the FC group. Corpora lutea (CL) obtained on day 15 were incubated for 6 h with corticosterone or ACTH. Corticosterone (1 μmol/l) significantly (P<0·05) inhibited progesterone secretion in the IC group but not in the FC group. The inhibitory effect of corticosterone in the IC group was completely blocked by co-addition of 1 μmol testosterone/l or 1 μmol oestradiol/l but not by 1 μmol dihydrotestosterone/l. ACTH (1 μg/l–1 mg/l) had no direct effect on progesterone secretion in either the IC or the FC groups, although ACTH apparently decreases progesterone secretion in vivo. Placentae obtained from rats of the FC group on day 15 were incubated for 24 h with or without ACTH (1 mg/l). The supernatant after placental incubation without ACTH significantly (P<0·01) increased progesterone secretion by the CL in both the IC and FC groups, and also eliminated the inhibitory effect of corticosterone in the IC group. The supernatant after placental incubation with ACTH also increased progesterone secretion in the FC group as effectively as the supernatant from the control incubation, but it had no effect in the IC group. It is concluded that corticosterone directly inhibits progesterone secretion by the CL, whereas the inhibitory effect of ACTH is mediated through the placenta. The results indicate that these inhibitory effects of corticosterone or ACTH are eliminated if the CL has been exposed to enough placental hormones before day 15 of pregnancy.