OBJECTIVE: This study evaluated the in vitro effects of cocaine on the binding characteristics of α-and β-adrenergic receptors from pregnant human myometrium. STUDY DESIGN: By means of membrane fractions from myometrium obtained from 26 women at term undergoing cesarean section, equilibrium binding assays were performed with tritiated dihydroergocryptine for a-adrenergic receptors and iodine 125-cyanopindolol for β-adrenergic receptors. Equilibrium competition curves were determined with and without cocaine. Results were compared by one-way analysis of variance. RESULTS: Cocaine inhibited β-adrenergic receptor binding (inhibition constant = 132 μmol/L) but had little effect on a-adrenergic receptor binding (inhibition constant = 1.63 mmol/L). Benzoylecgonine, a stable metabolite of cocaine, had no effect on binding to either receptor. CONCLUSION: Cocaine selectively inhibits myometrial β-adrenergic receptor binding. This may after the contractile equilibrium of the pregnant uterus and could explain, in part, the association of cocaine abuse with premature delivery. © 1993.