The effects of terbutaline, a sympathomimetic amine with predominantly beta 2-agonist properties, on cardiac automaticity and contractility were studied. For isolated rabbit right atria, terbutaline, 1 X 10(-9) to 1 X 10(-5)M, was significantly less potent than isoproterenol. The maximum increase in heart rate induced by terbutaline was 45 +/- 17 beats/min, that by isoproterenol, 120 +/- 9 beats/min. For canine Purkinje fibers, terbutaline had less effect on spontaneous rate than isoproterenol; maximum increases above control were 9.5 +/- 2.5 and 18.6 +/- 7.0 beats/min, respectively. For isolated feline ventricular muscle, terbutaline, 1 X 10(-9) to 1 X 10(-6)M, was significantly less potent than isoproterenol in increasing peak developed tension and the rate of tension development. Superfusion with Tyrode's solution containing terbutaline, 1 X 10(-7)M, plus graded concentrations of isoproterenol, 1 X 10(-9) to 1 X 10(-6)M, resulted in response which were less than those observed when isoproterenol alone was superfused. Maximal effects of isoproterenol plus terbutaline were equivalent to those of isoproterenol in its effect on cardiac contractility and automaticity and explain the clinical observation that terbutaline is less toxic than isoproterenol in its effects on cardiac rhythm and contraction when administered for the treatment of bronchial asthma.