The kinetics of activitydependent, extracellular alkaline transients, and the buffering of extracellular pH (pH_e), were studied in rat hippocampal slices using a fluorescein-dextran probe. Orthodromic stimuli generated alkaline transients up to 0.05 pH units that peaked in 273 ± 26 ms and decayed with a half time of 503 ± 43 ms. Inhibition of extracellular carbonic anhydrase (ECA) with benzolamide increased the rate of rise by 25 percent, doubled peak amplitude, and prolonged the decay three to four-fold. The slow decay in benzolamide allowed marked temporal summation, resulting in a severalfold increase in amplitude during long stimulus trains. Addition of exogenous carbonic anhydrase reduced the rate of rise, halved the peak amplitude, but had no effect on the normalized decay. A simulation of extracellular buffering kinetics generated recoveries from a base load consistent with the observed decay of the alkaline transient in the presence of benzolamide. Under control conditions, the model approximated observed decays after acceleration of the CO₂ hydration-dehydration reactions by a factor of 2.5. These data suggest low endogenous ECA activity, insufficient to maintain equilibrium during these alkaline transients. Disequilibrium implies a time-dependent buffering capacity, with a CO₂> / HCO₃-contribution that is small shortly after a base load. It is suggested that within 100 ms, extracellular buffering capacity is about one percent of the value at equilibrium and is provided mainly by phosphate. Accordingly, in the time frame of synaptic transmission, small base loads would generate relatively large changes in interstitial pH.