The propranolol content of Purkinje fibers associated with antiarrhythmic and electrophysiological actions of the drug both in vivo and in vitro was determined. The minimum effective tissue content of propranolol that consistently reversed a sustained ouabain-induced ventricular tachycardia in vivo after i.v. propranolol was between 6.7-11.1 .mu.g/g of tissue. Propranolol doses producing Purkinje fiber contents of less than 6.7 .mu.g/g failed to revert the arrhythmia but did produce significant .beta. blockade as evidenced by a lack of inotropic and chronotropic responses to 0.5 .mu.g/kg of isoproterenol. The in vivo minimum effective tissue content was produced in isolated Purkinje fibers perfused with Tyrode''s solution containing 1.7 .times. 10-6 M propranolol. In Purkinje fibers this concentration of propranolol depressed the rate of phase 4 depolarization previously enhanced by ouabain 2.1 .times. 10-7 M. Propranolol, 1.7 .times. 10-6 M, did not alter membrane responsiveness and only slightly accelerated repolarization at 5 min. Propranolol, 0.85 .times. 10-6 M, did not significantly depress the ouabain-enhanced rate of phase 4 depolarization but did attenuate the response to epinephrine through .beta. blockade. The initial direct action of propranolol in reverting a ouabain-induced ventricular tachycardia to a sinus rhythm in the dog is probably depression of automaticity.