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A technique is described for measuring regional blood flows in the brain of the cat. The measuring system is based on the principle of photoelectric plethysmography, a method commonly used to measure skdn blood flow. To miniaturize the photoelectric flow probe to a size compatible with cerebral insertion, a light emitting diode (LED) was used as the light source and a photo-Darlington array was used as the photodetector, the entire structure being imbedded in the tip of a 22 gauge needle. To test the flow needle, blood flows were measured in the cerebrum and hypothalamus of the cat during nembutal infusion and CO2 breathing and were then compared to the blood flow measured by an electromagnetic flowmeter placed on the carotid artery. The results of these experiments showed the feasibility of using an indwelling photoelectric flow needle for measuring regional blood flows in the cat brain.