The purpose of this study was to investigate the presence of a K(+)-H+ exchanger in brush-border membrane vesicles (BBMV) isolated from chick small intestine. 86Rb+, as a tracer for K+ transport, was used to probe for the exchange mechanism. An outwardly directed proton gradient (pH 5.5 inside, pH 7.5 outside) stimulated 86Rb+ uptake into voltage-clamped BBMV. H(+)-driven 86Rb+ uptake was only weakly inhibited by 5-(N-ethyl-N-isopropyl)amiloride, whereas this agent strongly inhibited H(+)-driven Na+ uptake. At initial rates, proton-driven 86Rb+ uptake was significantly reduced by external K+ but it was not significantly affected by external Na+. Conversely, extravesicular Na+ inhibited proton-driven Na+ uptake, whilst K+ had little effect. H(+)-driven K+ uptake tended to saturate with increasing external K+ concentrations and Lineweaver-Burk analysis of the data revealed a Km for external K+ of 2 mM. These findings are consistent with the presence of K(+)-H+ exchange activity in the chicken jejunal brush-border membrane.