We measured simultaneously the oxidative metabolic activity, monitored as the tissue fluorescence attribute to intramitochondrial NADH, the extracellular potassium level with ion-selective microelectrodes, and the focal extracellular electrical potential, of one site in intact cerebral cortex of cats. When the cerebral was stimulated by trains of repeated electric pulses applied either directly to its surface or to an afferent pathway, the corrected cortical fluorescence (F-R) declined indicating oxidation of NADH, the activity of extracellular potassium [K+]o increased, and the extracellular potential (Vec) shifted in the negative direction. When mild to moderate stimuli not exceeding 10-15 sec in duration were used, a 3-fold correlation was found between these three variables. The regression of F-R over either Vec, or over log [K+]o had a positive ordinal intercept. The results are in agreement with earlier suggestions 4,24,25,43,45,46 that (a) much but not all the oxidative metabolic response of cortex to electrical stimulation is expended in restoring disturbed ion balance; and (b) that sustained shifts of potential (SP) in response to repetitive electrical stimulation are generated by glia cells depolarized by excess potassium. The magnitude of SP shifts associated with a given elevation of [k+]o are smaller in cerebral cortex than in spinal cord48,49. The correlation of F-R with [K+]o breaks down when pathologic processes of either seizure activity or spreading depression set in. During paroxysmal activity [K+]o tends to remain confined below 10-12 mM, a level observed in non-convulsing cortex as well, but oxidation of NADH progresses beyond that seen in non-convulsing cortex as well, but oxidation of NADH progresses beyond that seen in non-convulsing tissue. This observation is hard to reconcile with the suggestion that excess potassium is a factor in the generation of seizures, at least of the type observed in this study. When [K+]o levels exceeded 10-12 mM, spreading depression invariably followed at least under the unanesthetized condition in these experiments. During spreading depression [K+]o levels rose to exceed 30 mM, sometimes 80 mM. NADH was oxidized during spreading depression to a level comparable to that seen in seizures. The observations are compatible with the suggestion13 that spreading depression occurs whenever the release of potassium into extracellular fluid is overloading its clearance therefrom.