Endocochlear potential (EP) in the guinea pig was investigated under the influence of nifedipine and calcium chelating agents. The area from the scala tympani to the scala vestibuli was perfused with control and test solutions. EP decreased gradually after perfusion with the nifedipine solution, when the concentration was higher than 100 ng/ml. At a concentration of 200 ng/ml, EP decreased from +70 mV to +14 mV. At 1000 ng/ml, it decreased from +70 mV to +54 mV and stayed at this level during perfusion. It began to return to the initial level when perfusion with the control solution was recommenced. Cochlear microphonics (CM) also decreased with 200 ng/ml of nifedipine, but less markedly than EP. EGTA (10 mM) decreased EP from +80 mV to +67 mV. This change, however, was only transient and EP returned to the control level. EDTA (4 mM) had no significant effect on EP. These results suggest that Ca ions play a role in maintaining EP in a manner similar to that by which Ca modulates Na-K pump activity of the marginal cells in stria vascularis.