A technique is presented for recording both local blood flow, by means of hydrogen clearance method, and neuronal activity of the cat's cortex using a single microelectrode. The requirements of electrodes for recording cerebral blood flow are different from those for recording extracellular spikes. Only electrodes within a small range of impedance (measured at 10 Hz) are suitable to record both. A bridge method is used for the recordings. The bridge signal is split in order to amplify cell activities and H2 responses separately. Three examples demonstrate the method and the importance of appropriately selected electrodes.