In a child with congenital secretory diarrhoea, jejunal perfusion studies showed net water and electrolyte secretion and suggested a defect in sodium/proton exchange. A technique was developed for investigation of such exchange in brush-border membrane vesicles from jejunal biopsy specimens. Vesicles from control specimens displayed proton-gradient-dependent sodium uptake, whereas those from the patient did not. These findings, as well as pointing to a cause of congenital secretory diarrhoea, confirm the existence and importance of jejunal sodium/proton exchange in man.