Intracoronary streptokinase (SK) is being used to treat acute myocardial infarction. Complications including atrioventricular (AV) block and both supraventricular and ventricular arrhythmias have been described. Selective cannulation of the nutrient arteries of the sinus node and aV node in the pentobarbital-anesthetized dog provides a model to study the specific pharmacologic effects of drugs administered directly to these structure. Selective perfusion of 2 ml normal saline into the sinus node artery of five dogs resulted in a transient 38.6% decrease in heart rate (148 ± 9.6 to 91 ± 12.7 bpm). This control response is typical of the method. Administration of 100, 500, 1000, 2000, and 4000 IU of SK/ml in these same dogs resulted in similarly brief bradycardias of 41.2%, 40.2%, 39.4%, 41.2%, and 40.3% (p = NS) below the control rate, respectively. The tachycardia induced by 0.0125 and 0.025 μg/ml of norepinephrine (24.2% and 31.4% sinus acceleration) was unaffected by the simultaneous administration of 500, 1000, and 2000 IU SK/ml. AV conduction was unchanged by selective perfusion of 1000, 2000, and 4000 IU SK/ml into the AV node artery of seven dogs, respective AV intervals being 111, 114, and 116 msec, with the control interval being 109 msec. Thus after selective perfusion of either the sinus node or AV node with SK in clinically used concentrations, we found no significant chronotropic or dromotropic effect in the dog. © 1984.