The effects of naloxone and propranolol on cardiac arrhythmias and durations from respiratory arrest to ventricular asystole and cardiac standstill were studied in unanaesthetized young rats induced to suffer respiratory arrest and exhibit ventricular fibrillation (VF) by a modified chloroform hypoxia technique. Both naloxone and propranolol reduced the incidence of VF dose dependently, indicating that they have antiarrhythmic effects. Addition of naloxone to propranolol shifted the dose-response curve of propranolol to the left significantly, indicating an additive effect of the two drugs in their antiarrhythmic activity. Both naloxone and propranolol prolonged the duration from respiratory arrest to ventricular asystole. However, joint administration of both did not further prolong this duration indicating an absence of additive effects. Naloxone prolonged the duration from respiratory arrest to cardiac standstill, indicating that naloxone prolonged the survival time. In contrast, propranolol did not produce the same effect. That naloxone both produced antiarrhythmic effect and prolonged the survival time whereas propranolol only corrected cardiac arrhythmias suggests that the antiarrhythmic effect of naloxone may not result in prolongation of survival time and that different mechanisms may be involved in the antiarrhythmic effect.