The ultrastructure of the normal human endolymphatic sac (ES) has been observed by transmission electron microscopy (TEM). The role of the epithelium, the various regions of the subepithelial space and the general anatomy of the ES in pressure regulation were morphologically studied to generate testable hypotheses of human ES function. Light microscopic (LM) and TEM evidence of pressure regulatory mechanisms by endolymph resorption, mechanical factors, and secretory activity are presented. These mechanisms may be useful in designing experimental studies of the ES, and in interpretation of retrospective LM and TEM studies of patients with Menière's disease.