It has been generally accepted that only beta adrenergic receptors are present in myocardial tissue, however, there are many reports of the existence of both alpha and beta receptors in mammal's hearts (1-4). In our laboratory also, existence of alpha adrenergic receptors was certified in rat atria (5): dose response curve of positive inotropic action of phenylephrine (PHE) shifted to the left after 6-propyl-2-thiouracil (PTU) treatment and depressed profoundly by phentolamine. In chronotropic response, alpha receptors have been minimally implicated. James et at. (6) reported that in dogs, methoxamine's negative chronotropic action is due to alpha receptor stimulation. The present paper concerns changes of sensitivity in positive chronotropic response to PHE and to isoproterenol. (ISO) after PTU treatment and adrenalectomy. Involvement of alpha adrenergic receptors in chronotropic response to PHE in rat atria are discussed.