We examined the effect of histidine-specific reagents on the transport activity of the Na+-H+ exchanger in microvillus (brush-border) membrane vesicles isolated from the rabbit renal cortex. Rose bengal-catalyzed photo-oxidation caused irreversible inhibition of the rate of Na+-H+ exchange but also caused significant loss of vesicle integrity. Treatment of the membrane vesicles with diethylpyrocarbonate caused inactivation of Na+-H+ exchange that could not be attributed to vesicle disruption or collapse of transmembrane H+ gradients. Inactivation of Na+-H+ exchange by diethylpyrocarbonate followed pseudo-first order kinetics to below 10% residual activity, could be reversed by hydroxylamine, was reflected by a decreased Vmax with no change in the Km for Na+, was dependent on external pH but not internal pH, was blocked by amiloride, and was enhanced by Na+. These data are consistent with the hypothesis that a diethylpyrocarbonate-sensitive imidazolium residue is the titratable group found in kinetic studies to bind H+ at the external transport site of the Na+-H+ exchanger.