Pharmacological analysis of the inhibitory effect of isoproterenol on the pregnanr and non-pregnant strips of rat uterus, and isoproterenot induced desensitization has been made on the basis of 봭-adrenergic receptors. 1. Inhibitory effect of isoproterenol in the pregnant uterus was greater than in the non-pregnant uterus, while the rate of spontaneous contraction was higher in the non-pregnant uterus. 2. Inderal inhibited the inbibitory effect of isoproterenol in both preparations, resulting in the parallel shifts of does responsive curves to the right. 3. Isoprotereno1-induced desensitization. 4. Isoprotereno1-induced desensitization was not prevented by the treatment of isoprotereno1, inderal or pyrogallol. From the above result, it is suggested that difference of inhibitory effect of isop roterenol in the non-pregnant uterus and in the pregnant uterus, and isoproterenol induced desensitization were not due to the alteration of β-adrenergic receptor.