In experiments on 100 cats with multichannel recording, the effects of three vasodilators - papaverine, chloracyzine, and bencyclane (Halidor) - on the volume velocity of the blood flow (by microthermistor technique) and on oxygen tension (by polarography) in the carotid artery, the cerebral cortex, the thalamus, and in white matter were tested. Considerable differences were found in the effects of the drugs mentioned, although all of them augment the total and local blood flow in the brain, and in most instances elevate the oxygen tension in arterial blood and cerebral tissue. Characteristic of papaverine is a uniform augmentation of blood supply to the cerebral cortex, the thalamus, and white matter, whereas chloracyzine, and especially bencyclane, primarily augment the blood supply to the cerebral cortex. In an analogous way the drugs tested influence the cerebral blood flow and oxygen tension in experimental cerebral ischaemia induced by intracarotid infusion of a serotonin solution.