Recording from single neurons in the human brain after head trauma was done through hair-like microelectrodes implanted during therapeutic operation. Types of responses were: (1) no unit activity; (2) unit activity in a random-like sequence; (3) multi-unit activity in synchronous rhythmic bursts and (4) unit activity at a more or less constant frequency. The method described here could be extended for use in any brain lesion where direct and quantitative postoperative information about the function of different neuronal populations is desired. This extremely sensitive test of neuronal function may be valuable in evaluating cerebral dysfunction in various clinical states, and as an indicator of imminent death. The latter assumes practical importance in patients who are potential donors of organs for transplantation. Continued research should confirm the value of this procedure.