Human placental cells maintained were used to determine the direct effect of beta-adrenergic agonists on progesterone accumulation by the placenta. In five studies, when either of the beta-adrenergic receptor agonists, terbutaline or isoproterenol, 1.5 micrograms/ml, was incubated for 24 hours with the placental cells, significantly more progesterone was present in the culture medium than in the medium of matched controls. Compared with the medium of matched controls, progesterone in medium increased 36 +/- 6% (mean +/- SE) when cells were treated with terbutaline and 38 +/- 7% when cells were treated with isoproterenol. In two dose-response studies with terbutaline, a minimal dose of 0.15 micrograms/ml was required to significantly increase progesterone in medium compared with that in matched controls. In three studies, propranolol at a concentration of 1.5 micrograms/ml completely blocked the stimulatory effect of 1.5 micrograms/ml of terbutaline.