SummaryWe measured cardiac output and its distribution (microspheres), blood gases and pH, heart rate, and pulmonary and aortic pressures in three groups of 1- to 3-day-old lambs.Group I consisted of six animals who had these measurements made during both control (normoxemic) and hypoxemic (PaO225 ± 3 mm Hg) periods. Hypoxemia increased the pulmonary artery pressure 70% (p<0.01). This elevation in pulmonary arterial pressure lasted as long as the animals were hypoxemic (90 min). Hypoxemia had no effect on cardiac output, mean systemic arterial pressure, heart rate, or the rate-pressure product of the left ventricle. Blood flow to the heart increased an average of 228% (p<0.05); flow to the brain increased 233% (p<0.05); flow to the skin decreased 35% (p<0.05) after 60 and 90 min of hypoxemia. Blood flow to the remaining organs was uneffected by hypoxemia.Group II consisted of four animals who were given 1, 5, and 10 mg/kg/h of tolazoline (Priscoline) intravenously while normoxemic. There was no effect on the pulmonary arterial pressures or blood gases and pH. Tolazoline (5 and 10 mg/kg/h) reduced systemic arterial pressures an average of 22% (p<0.05). Tolazoline (1 mg/kg/h) reduced blood flow to the spleen 48% and that to the brain by 20% and increased flow to the body 32% (p<0.05). Five mg/kg/h of tolazoline decreased renal and brain blood flow 35 and 20%, respectively (p<0.05) while that to the body and liver increased 26 and 48% (p<0.05). Tolazoline (10 mg/kg/h) returned splenic, body, and brain flow to control levels, and decreased renal blood flow further (48%) (p<0.05). Tolazoline had no effect on the left ventricular rate-pressureproduct.Group III was given 1, 5, and 10 mg/kg/h of drug while the animals were hypoxemic (PaO229 ± 4 mm Hg). The mean systemic arterial blood pressures decreased 20% with 5 and 10 mg/kg/h of tolazoline (p<0.05), but tolazoline did not change mean pulmonary arterial pressure. Tolazoline had no effect on the left ventricular rate-pressure products. Cardiac output increased 47% with hypoxemia, but it was unaltered by the addition of tolazoline. Blood flow to the heart increased 300% and that to the brain 200% with hypoxemia and remained at these levels with infusing tolazoline. Flow to the remaining tissues was constant. Five mg/kg/h of tolazoline increased cardiac flow further (560% of control). Ten mg/kg/h maintained the high cardiac flow seen with 5 mg/kg/h infusions of tolazoline, decreased that to the kidneys 33%, and increased flow to the lower body 67%. These data indicate that tolazoline does not decrease the pulmonary hypertension caused by hypoxemia and that tolazoline does little to alter the effect of hypoxemia on the distribution of blood flow.