The effects of hypoxemia on left ventricular myocardial blood flow, myocardial oxygen consumption, and contractile function were studied in 12 conscious newborn lambs 4-24 days after birth. Through a left thoracotomy, we placed fluid-filled catheters in the ascending aorta, coronary sinus, and left atrium. An electromagnetic flow transducer was placed around the ascending aorta, and a solid-state pressure transducer was introduced into the left ventricle. Three to four days later we measured aortic and coronary sinus blood oxygen contents, left ventricular myocardial blood flow, heart rate, aortic and left ventricular blood pressures, ascending aortic blood flow velocity, and the first derivative of left ventricular pressure (dP/dt) and ascending aortic blood flow velocity (dV/dt) during a control period and during 50 and 75% reductions in ascending aortic oxygen content. Myocardial oxygen consumption was calculated. There was no significant change in aortic or coronary sinus blood pH or CO2 tension during the study. Coronary sinus blood oxygen content and the arteriovenous difference of oxygen across the left ventricle decreased as a linear function of the aortic blood oxygen content. Myocardial blood flow increased in proportion to the reduction in aortic blood oxygen content. Myocardial oxygen consumption increased during hypoxemia but not as a function of aortic blood oxygen content. There was no significant change in left ventricular end-diastolic pressure or aortic mean blood pressure. dP/dt and dV/dt doubled during hypoxemia, but the increases did not occur as a function of the aortic blood oxygen content. In conscious, unanesthetized newborn lambs, 50 and 75% reductions of aortic blood oxygen content were associated with significant increases of left ventricular myocardial blood flow, oxygen consumption, and contractile function.