1. Experiments were made on single neurones and glial cells in the central nervous system of the leech to study the accumulation of K that occurs in the extracellular spaces around neurones as a result of impulse activity.2. The resting potential of a neurone is too insensitive to be used for the estimation of small changes in K concentration. The undershoot of the action potential, however, provided a reliable indicator of the K accumulation that occurs around a neurone during activity.3. After a single impluse the amplitude of the undershoot of a second action potential was decreased; the effect corresponded to a peak increase in K concentration of about 0.8 mM/l. immediately after the spike and declined exponentially with a time constant of about 100 msec. With trains of impulses the K concentration increased exponentially, again with a time constant of about 100 msec. The final value of K depended on the frequency and could build up to about double the normal concentration of 4 mM/l.4. The build-up of K was markedly reduced when the extracellular space surrounding a neurone was enlarged by removing its glial investment.5. Synchronous, repetitive activation of groups of neurones caused a slow depolarization of neighbouring glial cells in the C.N.S. of the leech, similar to that observed in amphibia and mammals. The change in glial membrane potential was also used to estimate the changes in K concentration and these values agreed with measurements derived from the undershoot.6. Increases of K concentration in the bathing fluid of the same order as those caused by neural firing markedly affected the frequency of ;spontaneous' neuronal discharges and synaptic potentials occurring within certain neurones in the C.N.S.7. The possible effects of physiologically occurring increases of K concentration on integration are discussed.