Changes of extracellular proton concentration ([H+]o) and K+ activity ([K+]o) were simultaneously measured by ion-sensitive microelectrodes in the CA3 region of guinea pig hippocampal slices. Repetitive electrical stimulation and application of glutamate or GABA were associated with prominent alkaline transients of up to 0.2 pH units lasting 2-10 s followed by smaller acid transients lasting up to 4 min. About 10-fold smaller alkaline transients were induced by spontaneous field discharges in the presence of bicuculline. The time to the maximal amplitude of the alkaline transients and the time to maximal increases of [K+]o were in the same range, concurring with the assumption that alkaline transients are due to a proton influx through cationic channels. However, spontaneous field discharges in low-calcium solution in which synaptic transmission is reduced were associated with acid transients of up to 0.02 pH units lasting 2-20 s. An alkaline transient was superimposed on the acid transient only when increases of [K+]o exceeded 1.5 mM. The effects of changing [H+]o on electrically evoked field potentials and spontaneous field discharges were studied in the range from pH 7.00 to 7.80. Electrically evoked field potentials were markedly depressed from pH 7.15 to 7.00 and enhanced from pH 7.60 to 7.80. The frequency of spontaneous field discharges in the presence of bicuculline significantly decreased by reducing pH from 7.40 to 7.30 and continuously increased from pH 7.40 to 7.80. In the same way, the frequency and the amplitude of spontaneous field discharges in low-calcium solution decreased from pH 7.40 to 7.15 and increased from pH 7.40 to 7.80.(ABSTRACT TRUNCATED AT 250 WORDS)