The effects of acutely increased right ventricular afterload induced by mechanical constriction of the main pulmonary artery or with alveolar hypoxia on function of the left ventricle were assessed in six and nine lambs, respectively (aged 1 to 3 days). Mechanical constriction of the main pulmonary artery in newborn lambs compromised left ventricular function with significant decreases in systemic blood flow and the peak first derivative of left ventricular pressure when considered simultaneously and as single variables. In contrast, alveolar hypoxia augmented left ventricular function with significant increases in systemic blood flow and the peak first derivative of left ventricular pressure when considered simultaneously or as single variables. Interaction appears to exist between the right and left ventricles during the newborn period. The mechanically increased afterload may have compromised left ventricular function by altering its end-diastolic size, inotropic state, or both. On the other hand, the augmented left ventricular function in the presence of hypoxia may have been due to an increase in inotropic background. The clinical implications in some infants with pulmonary hypertension and left ventricular dysfunction are raised.