In the CA1 area of the hippocampus of urethane-anaesthetized rats, the greatest Δ[K+]o and Δ[Ca2+]o evoked by repetitive fimbrial – commissural stimulation were always found in the pyramidal cell layer; but there were large increases in [K+]o over a wide range of depth, whereas a major fall in [Ca2+]o was localized almost exclusively to the level of the pyramidal layer. A sustained focal negative potential was also evoked by fimbrial stimulation; it resembled Δ[K+]o in time course and depth distribution and therefore probably reflected cellular depolarization caused by increased [K+]o. The close correlation between Δ[Ca2+]o and Δ[K+]o and the appearance of population spikes (especially in bursts of three to four spikes) indicate that pyramidal cell firing and corresponding K-outward and Ca-inward currents are mainly responsible for the accumulation of [Formula: see text] and the depletion of [Formula: see text]. In CA3 pyramidal areas, Δ[K+]o and Δ[Ca2+]o were comparable in magnitude and distribution to changes seen in CA1, but they occurred after a longer latency, and the major Δ[Ca2+]o had a longer duration, consistent with a more prolonged Ca2+ current.