Intestinal dipeptide carrier system has been shown in vitro to be involved in intestinal absorption of betalactam antibiotics. Given that efficiency of this transport system depends on a pH gradient (extracellular pH < intracellular pH) at the brush-border membrane of enterocytes, we assessed the effects of amiloride, a known inhibitor of the Na-H exchange, on the bioavilability of oral amoxicillin in eight normal volunteers. Following a single 10 mg oral dose of amiloride, the absolute bioavailability of oral amoxicillin turned out to decrease by 27% (p < 0.01). The extent of reduction of oral amoxicillin AUC appeared to significantly correlate (p = 0.005) with the extent of decrease in potassium renal excretion but not with variations in antibiotic renal clearance under amiloride effect. Such patterns seem to confirm in vivo the role of the Na-H exchange in betalactam absorption and to suggest the underlying regulatory function of intracellular Na concentration, the latter depending on the (NaK)-ATPase activity.