The myometrium of the rat has been found to contain both alpha 1- and beta-adrenergic receptors. To investigate the implication of these adrenergic receptors in uterine reactivity near term delivery, we have measured the number and the affinity of alpha 1-adrenergic antagonist [( 3H]prazosin: [3H]PRAZ)-binding sites and of very high affinity beta 2-adrenergic agonist [( 3H]hydroxybenzylisoproterenol: [3H]HBI)-binding sites in myometrial membranes throughout the last 5 days of pregnancy and at delivery. The number of specific binding sites was constant from Day 18 of pregnancy up to 6 h prior to birth. In the last 6 h of pregnancy, there was a sharp increase in the number of alpha 1-receptors (+70%, p less than 0.05). Simultaneously, the number of beta 2-receptors coupled to the adenylate cyclase system dropped (-75%; p less than 0.001). These results indicate that with the approach of parturition, there is a regulation of uterine reactivity by a modulation of the concentrations of myometrial adrenergic receptors during the last 6 h of gestation.