Localization of elements at the cellular and sub-cellular levels was performed with the energy dispersive X-ray microanalysis technique, using shock-frozen, freeze-dried and araldite-embedded mouse (CBA/CBA) cochleae sectioned dry. Anatomical identification occurred in the STEM (scanning transmission electron microscopy) mode. In inner hair cell stereocilia the K/Na ratio was 70:1 but only 20:1 in the cytoplasm. In outer hair cell cytoplasm the K/Na ratio was 11:1 while the ratio in stereocilia was similar to that in inner hair cells. Ca was identified in stereocilia and the upper part of the cytoplasm of both outer and inner hair cells. The elemental composition in the subtectorial space is endolymph-like and that in the inner tunnel of the organ of Corti is similar to extracellular fluid. Considerable regional differences in elemental composition occur in the tectorial membrane with regard to P, K and Ca. The highest concentration of Ca occurs in the basal part of the tectorial membrane towards the sensory hairs. The highest concentration of K occurs in the basal and outer parts whereas the middle part of the tectorial membrane contains low levels of both K and Ca. The elemental composition changes in two main directions: 1) from the limbal (growing) region to the tip of the tectorial membrane, and, 2) from upper to lower surfaces. The three cell types of the stria vascularis differ considerably in elemental composition. The highest concentration of K occurs in marginal cells. The basal cells contain more K than do the intermediate cells. A significantly higher concentration of Ca, Cl and Na occurs in marginal cell cytoplasm than in any other cell type in the stria vascularis.