The effect of CO2/HCO3- on the endocochlear potential (EP) was examined by using both ion-selective and conventional microelectrodes and the endolymphatic or perilymphatic perfusion technique. The main findings were as follows: (i) A decrease in the EP from similar to+75 to similar to+35 mV was produced by perilymphatic perfusion with CO2/HCO3--free solution, which decrease was accompanied by an increase in the endolymphatic pH (Delta pH(e), similar to 0.4). (ii) Perilymphatic perfusion with a solution containing 20 mM NH4Cl produced a decrease in the EP (Delta EP, similar to 20 mV) with an increase in the pH(e) (Delta pH(e), similar to 0.2), whereas switching the perfusion solution from the NH4Cl solution to a 5% CO2/25 mM HCO3- solution produced a gradual increase in the EP to the control level with the concomitant recovery of the pH(e). (iii) The perfusion with a solution of high or low HCO3- with a constant CO2 level within 10 min produced no significant changes in the EP. (iv) Perfusion of the perilymph with 10 mu g/ml nifedipine suppressed the transient asphyxia-induced decrease in EP slightly, but not significantly. (v) By contrast, the administration of 1 mu g/ ml nifedipine via the endolymph inhibited significantly the reduction in the EP induced by transient asphyxia or perilymphatic perfusion with CO2/HCO3--free or 20 mm NH4Cl solution. These findings suggest that the effect of CO2 removal from perilymphatic perfusion solution on the EP may be mediated by an increase in cytosolic Ca2+ concentration induced by an elevation of cytosolic pH in endolymphatic surface cells.