Respiratory alkalosis (hypocapnia) is used to treat persistent pulmonary hypertension in newborns. With the exception of the brain, there are no data on the effects of hypocapnia on organ blood flow in the neonate. Therefore, we examined the effects of 2 h of hypocapnia on blood pressure, oxygen consumption, cardiac output and organ blood flows in puppies.In six animals (group I), reducing the Paco2to 21.8 ± 1.5 mm Hg (pH 7.62 ± 0.04) caused an immediate and sustained reduction in cerebral blood flow (40%) and in myocardial blood flow (25%). There were no significant changes in arterial blood pressure, total body oxygen consumption, cardiac output, right and left ventricular rate-pressure product, or blood flow to the gut, liver, muscle, and kidneys.In four control animals (group II) (Paco239.8 ± 3.0, pH 7.38 ± 0.04), there were no changes in any of the measured variables (arterial blood pressure, total body oxygen consumption, cardiac output, or blood flow to any organ, including brain and heart) during 2 h of normocarbic ventilation.We did not determine whether the reductions in cerebral and myocardial blood flows were detrimental. We suspect that they were not because the animals did not develop metabolic acidosis and they had normal cardiac outputs, and ventricular rate-pressure products throughout the study.