From the clinical viewpoint, the patients with hypoxaemia have a high incidence of ventricular arrhythmias. However, there have been diverse results in animal models (Turnbull et al., 1965; Szekeres and Papp, 1967; Rogers et al., 1973; Murnaghan, 1975) possibly due to the effects of extracardiac factors such as neurohumoral reflex. To exclude such factors, we used an isolated perfused heart preparation to examine the direct effects of hypoxia on the arrhythmia. In this study, measuring the ventricular fibrillation threshold (VFT) in the isolated perfused rat heart, we examined the oxygen dependence of the susceptibility to ventricular fibrillation quantitatively under various conditions.