Equilibular disturbances are common in the elderly (1) and this decline in function probably indicates cellular deterioration. However, in humans it is difficult to separate the cellular changes that are age-related from those brought about by a lifetime of disease and antibiotic therapy. The establishment of a convenient animal model is important for many experiments relating balance organs and age. The aim of the proposed research is to describe the cellular changes occurring in the vestibular organs of the inner ears of C57BL/6 mice as a result of aging. Although the mouse is commonly used as a model for other types of inner ear research (2-6), no quantitative ultrastructural information is available on the aging mouse vestibular organs nor on genotype-specific progressive degeneration. The saccule, utricle, and semicircular canal ampullae of six age groups from weanling to very old will be compared to determine the temporal sequence oc cytological aging in this mouse strain. Standard morphometric and stereological techniques at the light and electron microscopic level will be used to describe the cytology of the sensory epithelium. Variation and significant changes will be analyzed at the following four levels: within an organ, among different organs in the same animal, among animals within an age group, and among the six age groups. A video image analysis system will be employed for semi-automated data collection and analysis.