The present experiments were performed to gain information on changes in various organ flow rates during i.v. infusion of vasodilating substances. The microsphere method with the reference sample technique was used in rats to measure simultaneously different organ flow rates. The detection of the spheres (non-radioactive) was performed by macerating the tissues and counting samples under the microscope. As only one microsphere-infection per animal was performed, an untreated experimental group served as a control. The cardiac output was measured in a separate series. The effects of isoproterenol, pentoxifylline, bencyclane, acetylcholine, and dihydroergotoxine (mixture of 3 compounds, Hydergin) consisted of flow increases in several organs while the flow in others remained nearly unchanged. Only during the action of sodium nitroprusside the flow values were found decreased or remained unchanged. This substance led to decreased cardiac output, whereas that in the other groups was unchanged or raised. The experiments point to quantitatively different reactions on cardiac output distribution during application of the vasodilating substances tested.