Regional blood flow measurements were made using the radioactive microsphere technique of Rudolph and Heymann [13] before Histamine and 3 times after the i.v. infusion of 2 mg/kg morphine in 9 unanesthetized monkeys restrained in horizontally tilted primate chairs. The dose was chosen after response curves established its minimal but consistent hypotensive effects. Although little systemic hemodynamic change was observed, the morphine evoked significant increases in blood flow to the heart, brain and bone, which lasted up to 11/2 h. The changes in regional blood flow outlasted the effects on blood pressure. Initially blood flow to the bronchial arteries was reduced to 50% of baseline. These changes were associated with depressed respiration, decreased pH and PO2 and an increased PCO2 of arterial blood. Arterial blood levels of histamine were not significantly changed after the morphine infusion; therefore, histamine cannot be considered the mediator of the changes in regional blood flow.