Levels of 19 free amino acids in cochlear endolymph and perilymph of scala vestibuli of the guinea pig were determined using reverse phase high performance liquid chromatography (HPLC) of the o-phthaldialdehyde-ethanethiol derivatives with fluorescence detection. Aspartate and glutamate were found to be significantly higher in endolymph than in perilymph, confirming results from another study using a different analytical method. The remaining 17 amino acids were significantly lower in the endolymph, in many cases by an order of magnitude. The data are compared with results on utricular endolymph and vestibular perilymph obtained by HPLC in another study from our laboratory. Possible implications and interpretations of the results are discussed.