Red blood cells have been shown to possess specific insulin receptors, with characteristics similar to the receptors of typical target cells. The present work was carried out to study the modulation of amiloride sensitive Na/H antiport by insulin on rat red blood cells. The activity of the Na/H antiport was determined by a new technique which involves the measurement of the fluorescence of 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) loaded rbc's as function of their intracellular pH (pHi). Our results show that the antiport in red blood cells displays the same behaviour as in other cells and it is inhibited by amiloride. Insulin stimulates the antiport with a dose-dependence similar to other typical insulin effects: a maximum at 10 nM and a smaller effect at higher and lower hormone concentrations. Insulin effect on the antiport was completely abolished by amiloride (0.1 mM) and significantly inhibited by ouabain (1 mM) showing, also in red blood cells, the strict dependence of the Na/H antiport on the functioning of the Na pump.