1. Short (2 s) trains of stimuli were applied to the dorsal hippocampal surface of cats, producing an increase in [K+]o and a decrease in NADH fluorescence (the latter being indicative of an increase in tissue oxygen utilization). 2. The [K+]o rose rapidly during stimulation (delta[K+]o values from 1 to 6 mM) with larger stimulus currents producing larger changes. The time course of the poststimulus decline of [K+]o was an exponential decay function, with T 1/2 values varying from 1.3 to 6.9s, and independent of the magnitude of the delta[K+]o. Consistent undershoots of [K+]o occurred following stimuli causing less than 1 mM change in [K+]o. 3. The maximum depression of fluorescence and the time integral of the fluorescence changes following each stimulus train were both highly correlated with the total increase of [K+]o occurring during the stimulus train. 4. Application of several stimulus trains in close succession resulted in more rapid potassium reuptake following the later trains and an unusually large undershoot after the last train. Concomitantly, there was a progressive decrease in the fluorescence level. 5. When afterdischarges were induced by prolonged (less than 2 s) stimulation, larger and more sustained increases in [K+]o and decreases of fluorescence were observed, and there was some indication that afterdischarges were followed by accelerated reuptake of extracellular potassium.