The present investigation was undertaken to study the effect of chronic treatment with atenolol on noradrenaline and terbutaline in hypertensive and hyperthyroid rats. The maximum increase in force of contraction of ventricle by noradrenaline in control preparations chronically treated with atenolol was reduced, while that produced by terbutaline was not affected. The pD2 values of both the agonists were increased. In DOCA-saline hypertensive preparations, chronic treatment with atenelol did not produce any change in noradrenaline-induced positive inotropic responses; however, the maximal response to terbutaline was reduced. While the pD2 value of noradrenaline was decreased, that of terbutaline was increased. Chronic treatment with atenolol in hyperthyroid rats did not produce any significant effect on the maximal contractions of ventricle with either noradrenaline or terbutaline. However, the pD2 value of noradrenaline was reduced while that of terbutaline was unaffected. It is concluded that the beneficial effects of atenolol in hypertension and hyperthyroidism may be related to reduction in the number of beta-receptors in the heart.