Extracellular pH changes were measured in the rat hippocampal slices using the pH-sensitive dye Phenol red. pH changes accompanied artificially evoked synaptic transmission in the dendrite area of dentate gyrus neurones and pyramidal neurones (CA1). Single electrical stimulation of presynaptic pathways produced a rapid acidic pH shift which was followed by a long-lasting alkaline one. The duration (nearly 10 ms) and amplitude of the acidic shift were closely related to the orthodromically evoked population excitatory postsynaptic potential. Population action potential, when elicited antidromically or in conditions of blocked synaptic transmission, did not produce any pH changes which are supposed to be specifically linked to the synaptic transmission.