A significant deterioration of heart function induced by aortic cross-clamping is reported. The aorta was clamped during various surgical procedures without any complication and the hemodynamic and metabolic variations induced by 20 min thoracic aorta cross-clamping in 6 open-chest dogs was studied. During clamping the most striking modifications were an increase in coronary blood flow of over 65%, an increase in cardiac work of over 77% and an increase in total peripheral resistances of over 68% compared to base values. No significant variations were found in the heart rate and cardiac index. Metabolic parameters, such as O2 consumption and lactate consumption increased significantly during the clamping period. The increase in lactate consumption and the progressive and continuous improvement in oxygenation indexes (lactate/pyruvate, redox potential, excess of lactate) proved the absence of any myocardial anoxia during this period. Within 15-30 min after declamping all hemodynamic and metabolic parameters reverted to values close to basal values. The mammalian heart apparently tolerates this procedure satisfactorily for a limited period of time. In clinical settings, the base conditions of the cardiovascular system should be considered before transposing these conclusions.