As a result of the normal electrical signaling of neurons, potassium accumulates in the narrow clefts separating the cellular elements of the nervous system. The increase in extracellular potassium, [K+]0 depends on the spatial and temporal pattern of electrical activity in the neurons and the removal of the accumulated K+ by diffusion, active transport, and current flow through cells. Increases in [K+]0 have been estimated indirectly by matching changes in nerve spikes and glial membrane potentials produced by activity with increases in [K+]0 bathing the preparation. Direct estimates have been made using K+-selective electrodes. Measurement with K+-selective electrodes and glial membrane potential have poorly defined spatial and temporal resolution; they indicate an "average" [K+]0 in the vicinity of the recording site. Under normal conditions elevated [K+]0 may modify the efficacy of synaptic transmission, vary rates of spontaneous spikes in neurons, increase sodium pumping in neurons, modify the sensitivity of receptors, and serve to coordinate neuronal activity with glial metabolism.