The gaseous compound carbon monoxide (CO) has been identified as an important endogenous biological messenger in the brain and is a major component in regulation of cerebrovascular circulation in newborns. CO is produced endogenously by catabolism of heme to CO, free iron, and biliverdin during enzymatic degradation of heme by heme oxygenase (HO). The present study was designed to test the hypothesis that endogenously produced CO contributes to hypotension-induced vasodilation of cerebral arterioles. Experiments used anesthetized piglets with implanted, closed cranial windows. Topical application of the HO substrate, heme-Llysinate (HLL), caused dilation of pial arterioles that was blocked by a metal porphyrin inhibitor of HO, chromium mesoporphyrin (CrMP). In normotensive piglets (arterial pressure 64±4 mm Hg), CrMP did not cause vasoconstriction of pial arterioles but rather a transient dilation. Hypotension (50% of basal blood pressure) increased cerebral CO production and dilated pial arterioles from 66±2µm to 92±7µm. In hypotensive piglets, topical CrMP or i.v. SnPP decreased cerebral CO production and produced pial arteriolar constriction to normotensive diameters. In additional experiments, since prostacyclin and nitric oxide (NO) are also key dilators that can contribute to cerebrovascular dilation, we held their levels constant. NO/prostacyclin clamp was accomplished with continuous, simultaneous application of indomethacin, N-nitro-L-arginine (L-NNA), and minimal dilatory concentrations of iloprost and sodium nitroprusside (SNP). With constant NO and prostacyclin, the transient dilator and prolonged constrictor responses to CrMP of normotensive and hypotensive piglets, respectively, were the same as when NO and prostaglandins were not held constant. These data suggest that endogenously produced CO contributes to cerebrovascular dilation in response to reduced perfusion pressure. KEYWORDS: heme oxygenase, autoregulation, pial arterioles, newborn.