Abstract:α‐And β‐adrenergic inotropic effects have been shown to be qualitatively different. In order to further characterize these differences we compared the mechanical responses to α‐ and β‐adrenoceptor stimulation, respectively, in electrically driven left ventricular papillary muscles from rat heart. The muscles were stimulated by either isoprenaline (β‐adrenoceptor stimulation), phenylephrine in the presence of propranolol (α‐adrenoceptor stimulation) or phenylephrine alone (combined α‐ and β‐adrenoceptor stimulation). Isometric tension (T), rate of rise and decline of tension (first derivative = T′) and rate of transition from tension rise to tension decline (negative part of second derivative = T″) were recorded. These recordings disclosed qualitative differences between the α‐ and β‐inotropic response both in dose‐response and time course experiments. Maximal β‐adrenoceptor stimulation caused a small increase in Tmax(18%), intermediate increases in T′max(45%) and T′min(68%) and a considerable increase in T″min(145%) (“β‐type” effect). Maximal α‐adrenoceptor stimulation increased all qualities by about the same degree (23–24%) (“α‐type” effect). While β‐adrenoceptor stimulation gave a dose‐dependent and pronounced increase in the ratio T″min/T′max(relaxation‐onset index), α‐adrenoceptor stimulation decreased it to subcontrol values and phenylephrine alone gave a small dose‐dependent increase at higher doses. The time course of the α‐adrenoceptor stimulation was characterized by a transient decrease in all qualities followed by an increase which reached maximum at 4–5 min. β‐Adrenoceptor stimulation gave a monophasic response which reached maximum after 1–2 min. Phenylephrine alone gave mainly an “α‐type” effect although T″minincreased significantly more in the absence than in the presence of propranolol and T″min/T′maxshowed a small increase which developed slowly. Thus β‐adrenoceptor stimulation activated relaxation compared to contraction by a higher degree than did α‐adrenoceptor stimulation. This probably reflects different mechanisms of action. While the α‐effect may rely primarily on an i