The beta-adrenoreceptor antagonist [3H]-dihydroalprenolol (DHA) has been used to label adrenoreceptors in membranes from human pregnant myometrium and placenta. Six tocolytic drugs were tested for their ability to bind to the beta-adrenergic receptor of placental and myometrial membranes. Tocolytic agents competed with [3H]-DHA binding in the following order of potency: clenbuterol greater than fenoterol greater than ritodrine greater than isoxsuprine greater than orciprenaline greater than terbutaline. All drugs competed for [3H]-DHA binding sites with HILL coefficients greater than unity indicating heterogeneity of binding. In general, binding of beta-adrenoreceptor agonists was weaker in myometrium than in placenta homogenates. According to our results it is very likely that the placenta may play an important role in the pharmacological mechanism of tocolytic agents in inhibitory of premature labor.