The Ca2+ concentrations in cochlear perilymph and endolymph of the guinea pig were measured with double-barreled Ca2+-selective microelectrodes and showed 1.76 +/- 0.74 X 10(-3) M and 2.20 +/- 0.19 X 10(-5) M, respectively. The electrochemical potential gradient for Ca2+ between perilymph and endolymph was 23.2 mV and the existence of an active transport mechanism from the former to the latter was suggested. Vanadate given perilymphatically decreased the Ca2+ concentration in endolymph with a slight elevation of the endocochlear potential and was suspected of blocking the active transport. The Ca2+ concentration in endolymph was abruptly increased by anoxia or the intravenous administration of 60 mg/kg furosemide and was slightly increased by the intravenous administration of 30 mg/kg furosemide or 100 mg/kg acetazolamide. The endolymphatic pH measured with pH-microelectrodes under various conditions indicates that the mechanism of increase in the Ca2+ concentration is attributed not to the liberation of Ca2+ from the surrounding tissues caused by a fall in pH but to the increased influx of Ca2+ from perilymph due to the depression of the endocochlear potential.