To explore the sequence of metabolic, mechanical, and electrocardiographic (ECG) events during myocardial anoxia, isolated rat hearts were paced from the atrium. Anoxic and recovery periods were studied. Adenosine triphosphate (ATP) and creatine phosphate (CP) declined to 50% of control during the first minute and remained at that level for the 5-minute anoxic period. ATP and CP returned to control values after 10 and 20 seconds of recovery. Lactate and potassium efflux from the myocardium closely followed the highenergy phosphate changes. During anoxia, left ventricular systolic pressure increased initially, then fell below the control level after 2 minutes, and recovered within 20 seconds of reoxygenation. In catecholamine-depleted hearts, it fell immediately with anoxia, and recovery was incomplete. The conduction time for the pacing stimulus to reach the ventricle increased with anoxia and decreased with reoxygenation. S-T alterations in the ECG also lagged behind high-energy phosphate reduction and recovery.The study demonstrates that in the isolated heart, ECG evidence of myocardial hypoxia may be absent when high-energy phosphate levels in the myocardium are very low. Mechanical changes are more closely related temporally to high-energy phosphate alterations than are ECG changes. The release of endogenous catecholamines is important to maintain mechanical function in the hypoxic heart.