Total calcium (Ca) concentration in inner ear fluids was determined by fluorimetry, emission and absorption spectrophotometry, and electron probe analysis. The ionized Ca was measured with selective microelectrodes. In perilymph, the total Ca concentration (1.2 mM) was similar to the ultrafiltrable Ca concentration in plasma. The fraction of ionized Ca was 80%. In endolymph, a total Ca concentration of 0.5 mM contrasted with a reported ionized Ca concentration of 0.02 mM, which suggests, as a working hypothesis, that most of the Ca could exist as bicarbonate and/or phosphate undissociated salts. The decrease in the endocochlear potential induced an increase of the ionized fraction of the Ca. The electrochemical potential of Ca across the perilymph-endolymph barrier implies an active entry of Ca into the endolymph.