To estimate the relative contributions of pre- and postsynaptic elements, and of synaptic and action potential-related currents, to the elevation of interstitial potassium ([K+]o) that occurs during neural activation, we measured [K+]o and focal electrical potential (Vec) during ortho- and antidromic stimulation, before and after blocking synaptic transmission, in the CA1 region of hippocampal tissue slices in vitro. Single stimulus pulses could cause delta [K+]o as large as 0.25 mM in stratum (st.) pyramidale and 0.27 mM in st. radiatum; stimulus trains could cause delta [K+]o as large as 10.5 mM in st. pyramidale and 6.25 mM in st. radiatum. Stimulus trains also caused negative Vec shifts; these shifts were related in a linear fashion to delta [K+]o. For a given increase in [K+]o, the change in Vec was greater in st. radiatum than in st. pyramidale. In st. pyramidale, the delta [K+]o evoked by antidromic stimulation was 65% of the delta [K+]o evoked by orthodromic stimulation (with equal population spike amplitudes). Blockade of synaptic transmission by removal of Ca2+ reduced orthodromically evoked delta [K+]o in st. radiatum by 52%; delta [K+]o in st. pyramidale was abolished. Removal of Ca2+ caused an 11% decrease in the delta [K+]o evoked in st. pyramidale by antidromic stimulation. We conclude that in the layer of dendritic trees (st. radiatum), approximately half of the K+ ions released into the interstitial space during orthodromic stimulation come from presynaptic terminals, with the remainder probably resulting from the ion currents of postsynaptic potentials. Among the pyramidal cell bodies (st. pyramidale), almost all of the excess K+ is released by action potential currents.