The regional cerebral blood flow (rCBF) and cardiac output (CO) were measured in conscious rats by the microsphere method during control, after 5 and 60 min at 5 bar O2, and 5 min after decompression to air. The arterial acid-base balance was essentially unchanged during hyperbaric O2 and after decompression, except for a slightly reduced CO2 and HCO3 during the O2 exposure. The heart rate (HR) fell at 1 bar O2, continued to fall during compression, and remained low. A marked HR rise occurred in air after decompression. The systolic arterial pressure (AP) increased, while mean AP was constant during the O2 exposure. The CO and total cerebral blood flow fell in proportion to the arterial O2 content increase. The rCBF was unevenly distributed in control, and fell to a disparate degree and remained low in some regions during O2 exposure. Due to the rCBF fall, the O2 supply was limited, the glucose supply was reduced, and CO2 and heat transport probably were limited, suggesting a labile metabolic state locally in the brain. After decompression, blood flow remained low in several regions, making hypoxia likely for a considerable time in several brain areas, whereas the rest of the brain had normalized or increased blood flow. The HR and systolic AP remained high for at least 30 min after decompression.