Right and left ventricular outputs, pulmonary and systemic blood flows, and blood flows in both directions through the ductus arteriosus were measured before and during positive-end-expiratory-pressure in 5 premature lambs with induced hyaline membrane disease. During positive-end-expiratory-pressure, right ventricular output increased in all lambs without any significant change in left ventricular output or pulmonary vascular resistance. Left-to-right ductus flow decreased in lambs which initially had large left-to-right ductus shunts. significant right-to-left ductus flow did not occur before or during positive-end-expiratory-pressure.