Extracellular recordings were made from dendritic and somatic sites in the CA1 region of guinea pig hippocampal slices maintained in vitro. We studied the effects of increasing extracellular K+ from 3.25 to 15.25 mm on potentials elicited by synaptic activation of stratum radiatum fibers. Increasing K+ from 3.25 to 12.25 mm had little or no detectable effect on the excitability of presynaptic fibers, increased the amplitude of the somatic response (the population spike), and increased the size of the dendritic field EPSP in the range of 3.25 to 9.25 mm. When epileptiform activity induced by penicillin was studied as a function of [K+]0, it was found that elevating K+ increased the amplitude and duration of the abnormal population spike. However, significant epileptiform activity was present in 3.25 mm [K+]0. All evoked activity was reversibly abolished by perfusion with 15.25 mm K+. These results indicate that [K+]0 has an important role in regulating neuronal excitability and may effect changes both pre- and postsynaptically. © 1981.