Simultaneous monitoring of tissue O2 and regional cerebral blood flow (rCBF) was performed in the striatum of freely-moving rats. Differential pulse amperometry and constant potential amperometry were used to monitor O2 levels at a carbon paste electrode (CPE), while rCBF values were obtained using the H2 clearance technique. Two forms of behavioural activation were studied and the resultant changes in tissue O2 and blood flow compared. Both tail pinch and induced grooming produced immediate and parallel increases in O2 and blood flow which returned to baseline on cessation of activity. These findings indicate that under conditions of physiological stimulation the direct voltammetric measurement of O2 in brain tissue with a CPE can be used as a reliable index of increases in rCBF, resulting in an improvement in time resolution from 5 min (H2 clearance) to <1 s (amperometry). Because tissue O2 is a balance between supply by the blood stream and utilisation by the cells, increases in O2 current are an index of increased blood flow only when supply significantly exceeds utilisation.