Response of a hypoxic and acidotic (HA, with exogenous lactate) ventricular muscle tissue to subsequent reoxygenation in the absence of substrate (0 mM dextrose) was different from that of a Purkinje fiber. The K+ concentration in this solution (4.6 mM) was slightly higher than that in Tyrode solution (2.7 mM). The observed effects of reoxygenation of such a ventricular tissue were also variable. The ventricular muscle tissue exhibited the following different responses on reoxygenation after hypoxia and acidosis: (1) arrhythmias, without much depolarization of the membrane potential, (2) oscillatory after-potentials (OAPs) during the late diastole, which lessened in amplitude as the time of reoxygenation increased, but no arrhythmias, or (3) a pronounced slowed phase of repolarization (hump), but no arrhythmias. These different effects of reoxygenation did not occur if concentration of K+ in HA was very much higher than 4.6 mM. Common to these three different responses was the prolongation of the action potential durations during reoxygenation at the 50% and 90% levels of repolarization (APD50 and APD90) and a slight increase in the resting tension after 30-40 minutes of reoxygenation. Some of the observed responses of ventricular muscles were well mimicked by increasing extracellular calcium, but the different and variable effects of arrhythmias, OAPs, and prolonged APD require further analysis.