The effects of hypoxia on the conduction of excitation in ventricular muscle were examined. Transmembrane action potentials and isometric tension of isolated rabbit papillary muscle were monitored simultaneously. When the preparations were exposed to hypoxia, the action potential duration (APD) shortened progressively and conduction velocity, assessed by the interval between two action potentials, decreased appreciably. This conduction delay was always accompanied by an increase in resting tension (RT). The upstroke velocity of the action potentials, however, was virtually unaffected by 60 min of hypoxia. When the perfusate was reoxygenated, the APD, conduction velocity and RT recovered. The conduction delay and elevation of RT due to hypoxia were prevented by verapamil or a low [Ca]0 medium and were potentiated by isoproterenol. These results suggest that a cytoplasmic Ca++ accumulation, resulting in intercellular electrical uncoupling, may contribute to the conduction delay during hypoxia.