The antiarrhythmic effects of the calcium blocker verapamil and the beta adrenoreceptor blocker propranolol were examined in aminophylline toxic dogs. Eighteen dogs were intubated and ventilated after induction of anaesthesia (pentobarbitone 30 mg/kg and paccuronium 0.1 mg/kg). All animals were rendered toxic by aminophylline infusion; an initial dose of 50 mg/kg over five minutes with subsequent doses of 10 mg/kg over 30 seconds. Twenty minutes after each aminophylline infusion, the dog was challenged with phenylephrine (10 to 20 micrograms/kg). This resulted in short duration hypertension and reproducible emergence of ventricular arrhythmias. The dogs were divided into three groups of six animals each. Group I (control) received no antiarrhythmics whereas Group II received verapamil 0.2 mg/kg and Group III received propranolol 0.1 mg/kg for the treatment of persistent ventricular arrhythmias. Both verapamil and propranolol exerted an antiarrhythmic effect in aminophylline induced ventricular arrhythmias. The efficacy of verapamil was independent of the accompanying reduction in blood pressure and systemic vascular resistance as subsequent phenylephrine induced hypertension could not reinstitute these arrhythmias. Propranolol appeared less effective since it did not completely suppress the arrhythmias in three dogs and could not prevent emergence of PVC's in four following repeat phenylephrine challenge. Further development of this animal model may be useful for the better understanding of ventricular arrhythmias.