A method of measuring local cerebral blood flow, on the basis of inhalation of hydrogen gas and subsequent recording of local clearance curves of hydrogen by means of platinum electrodes, has been tested under different experimental conditions. The values of blood flow recorded from the caudate nucleus in awake cats, during barbiturate anesthesia, and during hypercapnia, are in agreement with previously published results. The preservation of the reactivity of cerebral vessels to CO2, and to changes in perfusion pressure (autoregulation), indicates that the vessels around the electrode tip maintain their physiological reactions. Further validation of the method is based on a direct comparison with blood flow values in the same structure, measured at short intervals of time by means of the autoradiographic method of Reivich and co-workers. No significant difference has been found in the average blood flow values recorded by the two methods. The hydrogen method therefore is reliable for measuring in acute experiments the local blood flow in subcortical structures of the brain.