Twenty-one patients have been studied in the 48 h after valve replacement to determine the possible contribution of abnormalities of left ventricular myocardial blood flow and oxygen consumption to the impaired cardiac performance which is sometimes evident in such patients. In the 14 patients making an uneventful recovery (Group A) the mean values for blood flow and oxygen consumption were both higher than in resting man, while the arterio-coronary sinus oxygen content difference was narrowed, with a high coronary sinus oxygen tension. Five patients had a low cardiac output (Group B) and had similar levels of blood flow and oxygen consumption to Group A, while the coronary sinus oxygen content and tension were reduced. When the heart rate was increased by pacing (Group C) myocardial oxygen consumption increased but coronary blood flow failed to rise, while the arterio-coronary sinus oxygen content difference widened slightly. It is concluded that the low postoperative cardiac output is not due to low coronary blood flow or myocardial oxygen supply, but these patients have a limited ability to increase their already high coronary blood flow. Therefore any increase in oxygen demand may be met by the potentially detrimental mechanism of widening the arterio-coronary sinus difference, with lower coronary sinus and tissue oxygen tension.