The effect of HCO 3 on ion absorption by young corn roots was studied in conditions allowing the independent control of both the pH of uptake solution and the CO2 partial pressure in air bubbled through the solution. The surface pH shift in the vicinity of the outer surface of the plasmalemma induced by active H § excretion was estimated using the initial uptake rate of acetic acid as a pH probe (Sentenac and Grignon (1987) Plant Physiol. 84, 1367). Acetic acid and orthophosphate uptake rates and NOf accumulation were slowed down, while S6Rb+ uptake and K § accumulation rates were increased by HCO 3 . These effects were similar to those induced by 4-(2-hydroxyethyl)-lpiperazineethane sulfonic acid/2-amino-2-(hydroxymethyl)-l,3-propanediol (Hepes-Tris). They were more pronounced when the H + excretion was strong, were rapidly reversible and were not additive to those of Hepes-Tris. The hypothesis is advanced that the buffering system CO2/H2CO3/ HCO3 accelerated the diffusion of equivalent H § inside the cell wall towards the medium. This attenuated the surface pH shift in the vicinity the plasma membrane and affected the coupling between the proton pump and cotransport systems.