WHEN RECORDED intracellularly in normal cerebral cortex of cat, cells are encountered that exhibit large membrane potentials (up to 90 mv), yet show no injury discharge when impaled and no spontaneous firing.1 Repetitive stimulation of either cortical surface or thalamus is capable of partially depolarizing (5-20 mv) these "idle" cells, but the depolarizing responses never give rise to all-or-none firing.2 Glia grown in tissue culture respond similarly to electrical stimulation and for this reason it has been suggested that "idle" cells are glia.3,4 Such results2-4 prompted us to do intracellular recording in glial tumors produced in mice by intracerebral implantation of 3-methylcholanthrene. The carcinogen also produces mesynchymal tumors (ie, sarcomas), and, therefore, we were able to compare membrane potentials of these tumors with those of glial origin. To our knowledge, intracellular potentials from cerebral neoplasms, experimentally induced or occurring pathologically in man, have not been