Extracellular pH transients were evoked in rat hippocampal brain slices by activation of a monosynaptic inhibitory pathway following pharmacological blockade of glutaminergic transmission. Repetitive stimulation in stratum radiatum near the recording site in stratum pyramidale evoked an immediate alkaline shift which was potentiated by pentobarbital and blocked by picrotoxin but not by 2-hydroxy-saclofen. Benzolamide, a poorly permeant inhibitor of carbonic anhydrase (CA), and prontosil-dextran 5000, a macromolecular CA inhibitor, abolished the alkaline transients evoked by stimulation and by exogenous GABA. Thus an extracellular CA is involved in regulating interstitial pH in brain, and the stimulation-induced alkaline transients are caused by net influx of CO2 into CA1 neurons in response to efflux of bicarbonate across postsynaptic GABAA receptor channels.