Regional distribution of brain blood flow was examined in 6 healthy nonmedicated swine during inhalation of 50% O2 (+ 50% N2) and at 45 minutes of 50% end-tidal nitrous oxide administration. All animals were surgically prepared 10 to 12 days before the hemodynamic study. Catheters were implanted in the left atrium, ascending aorta, descending aorta, and pulmonary artery. Brain blood flow was determined, using 15-micron diameter radionuclide-labeled microspheres injected into the left atrium. Administration of 50% nitrous oxide markedly increased blood flow in all regions of the brain (except corpus callosum), even though the animals were not excited and the arterial blood pressure, arterial blood-gas tensions, pHa, and cardiac output were not different from respective control values. At 45 minutes of 50% nitrous oxide administration, cerebral, cerebellar, and brain-stem blood flows were 144%, 137%, and 137% of respective control values. It is concluded that 50% nitrous oxide administration caused marked vasodilatation in all regions of the porcine brain.