In a series of 29 experiments in cats, the vasodilatory effect of Bencyclane on pial arterioles was investigated by means of the cranial window technique, using an image-splitting eyepiece, a photometric method or simple microscopic observation. Intravenous injection of 3 mg kg–1 led to vasodilatation in all experiments, yet decreased blood pressure within 30–40 sec until 5–6 min down to 70% of resting pressure. Mean maximal dilatation of arterioles with a 76-µm mean resting diameter was 53%. After normalization of blood pressure, arteriolar diameters remained increased by 5–10% for further 10 min, thus indicating increased cerebral blood flow for a total time of about 15 min. During intravenous infusion of 0.2–0.3 mg kg–1min–1 of the drug, pial arterioles dilated by about 10% with blood pressure remaining on resting levels. A higher dosage rate of infusion evoked further vasodilatation, yet parallel decrease of