Experiments were performed in order to determine whether progesterone secretion in the ovine placenta can be short-term regulated. There was an increase in progesterone content per unit weight in ovine fetal cotyledons as gestation progressed: 17.0 +/- 4.7 ng/100 mg of wet tissue in ewes between 40 and 54 days of pregnancy (n = 7) and 70.7 +/- 18.8 (n = 9) between 100 and 118 days. At all stages of pregnancy, neither progesterone nor 20 alpha-dihydroprogesterone synthesis were significantly affected when fetal cotyledons were incubated for 3 h in the presence of LH, 8-Br-cAMP, GnRH agonist or GnRH antagonist. Addition of pregnenolone to the incubation medium increased progesterone secretion in a dose-dependent manner while addition of 25-hydroxycholesterol did not. These results suggest that the existent (basal) synthesis of progesterone reflects the maximal capacity of steroidogenesis through the cholesterol side-chain-cleavage system. In the presence of these precursors, LH, 8-Br-cAMP, the phorbol ester derivative PMA and calcium ionophore A23187 were not able to modify progesterone or 20 alpha-dihydroprogesterone synthesis. These results also suggest that LH or GnRH and the two signal mechanisms involved in their action, i.e. cAMP and Ca2+ sensitive-inositol phospholipid-dependent mechanisms are not implicated in the short-term regulation of progesterone synthesis in the ovine placenta.