The spontaneous uterine motor activity of the post-partum rat was investigated in parallel with the in-vitro determination of the density of the alpha1 and beta-adrenergic receptors of the myometrium. The in-vivo experiments were performed by an improved method, using a Millar catheter fitted with a latex microballoon. The spontaneous contractility of the post-partum rat uterus was found to be highest 24 h after delivery, indicating that this time is the most suitable for pharmacological examinations of tocolytic agents. A very close correlation was found between the results of the in-vivo experiments and the alpha1/beta-adrenergic receptor ratio assessed by an in-vitro receptor assay, thus indicating that the state of the adrenergic receptor system fundamentally determines the contractility of the uterus. This conclusion is supported by the fact that the pharmacological sensitivity of the rat uterus to prazosin and fenoterol changed as a function of the post-partum time in accordance with the alpha1/beta-adrenoceptor ratio. These results and the relevant data available reveal a crucially important role of an alpha1-adrenoceptor-mediated process, implicating alpha1-blockers as theoretically potent agents for inhibition of premature uterine contractions.