Simple hydrogen-sensitive polarographical electrodes of thin platinum wire were inserted into the torcular Herophili of Rhesus monkeys. Hydrogen was administered by inhalation for ten minutes, after which the hydrogen clearance was recorded from torcular blood. At a Paco2of 32 mm Hg (SD ± 2.3), flow in the fast flow compartment was 102 ml/100 gm per minute (SD ± 19.1), and flow in the slow flow compartment was 28 ml/100 gm per minute (SD ± 5.8). Mean total cerebral blood flow was 52 ml/100 gm per minute (SD ± 10.5). Coefficient of variation was less than 10%.Our experience suggests that one may reliably measure average total cerebral blood flow in the experimental setting by following the clearance of hydrogen from torcular blood. The method is relatively simple, inexpensive and radiation-free. It can be easily combined with the standard hydrogen clearance technique for measuring local tissue blood flow, thereby permitting the simultaneous recording of both local and total brain blood flow.