Platinum microelectrodes were used to measure H2 clearance in mouse brain, and the clearance curves were used to calculate regional cerebral blood flow (rCBF). The curves were usually biexponential, whether or not electrode placement was confined to the cortex. When calculated by the height over area method, rCBF in anesthetized mice averaged 37+/-14 and 49+/-15 ml/100 gm per minute in two successive groups where cortical placement had been made. After CO2 breathing, which raised PaCO2 to 77+/-18 torr, the mean rCBF of the latter group was elevated to 70+/-36 ml/100 gm per minute. Our basal rCBF values are lower than literature values for rats or mice, when compared with data obtained by other techniques. However, our data are comparable to rat rCBF data obtainted by others using H2 electrodes and are comparable also to data for whole brain CBF obtained by a variety of methods in larger anesthetized mammals. It is possible that H2 electrodes provide low values for supposedly "cortical" rCBF in the very small mouse brain, because is such brains the electrode is usually close enough to a slow clearing compartment for the electrode reading to be influenced by that compartment. At the same time one cannot rule out the possibility that other techniques when applied to small rodents may, on occasion, produce spuriously high values for CBF. Indeed, while some studies using the latter techniques do show unusually high values for cortical flow in these animals, other studies using similar methods do not.