THE PRESENT experiment was undertaken to investigate those extensions to the impedance method of locating brain structures necessary to permit its use safely on humans. In previous reports1-3it was shown that a wide variety of brain structures can be identified with a roving electrode if the impedance of the tissue surrounding the electrode tip is monitored as the electrode is inserted into and through the brain. Before use on man, several factors relating to the necessarily high requirements of safety and of accuracy required systematic investigation. These factors are given below along with the results of the investigation. Method Animals and Tracks.—Data were obtained from 60 electrode tracks in tenMacaca mulatta. Six areas for exploration were selected and were the same for each brain, facilitating comparisons. Of the ten animals, six were anesthetized with 50 mg/kg pentobarbital. The other four were killed with an overdose of