The effect of aging on axosomatic synaptic terminals in the rat medial nucleus of the trapezoid body was studied using quantitative electron microscopy. In young adult rats (3 months of age), the mean percentage of the surface area of principal cells covered by synaptic terminals is 61.7% (S.E.M. = 4.1) while in aged animals (27-33 months of age) the per cent coverage is 43.7% (S.E.M. = 3.3). Likewise, between 3 and 27-33 months of age, the average number of synaptic terminals present along a 100 micron length of principal cell surface decreases significantly (P less than 0.001) from 28.3 (S.E.M. = 1.3) to 18.9 (S.E.M. = 1.3). Only terminals derived from calyces of Held are lost in the aged animals, displaying a 37% reduction between 3 and 27-33 months of age. The length of apposition by synaptic terminals in the medial nucleus of the trapezoid body does not change significantly with aging. We conclude that because of a significant loss of calycine synaptic endings, the structure of calyces of Held becomes less complex with advancing age in rats. This would presumably result in an age-related partial deafferentation of principal cells, causing significant alterations in the processing of auditory information in the medial nucleus of the trapezoid body.