Studies on white rats showed that intraperitoneal administration of small doses of fentanyl (0.005 mg/kg) and morphine (1 mg/kg) decreased local blood flow and increased partial pressure of oxygen (pO2) in the frontoparietal area of the cerebral cortex but had the opposite effects in the nucleus accumbens--where there was a significant increase in local blood flow and just as significant a decrease in pO2. Analysis of the data led to the conclusion that these changes must result from significant changes in functional-metabolic activity in these structures, induced by intraperitoneal administration of fentanyl or morphine.