The medial nucleus of the trapezoid body (MTB), a cell group in the superior olivary complex, was examined in an age-graded series of rats for neuron loss, changes in the giant synaptic endings (chalices of Held) on MTB neurons, and accumulation of age pigment. Neuron counts were done on protargol-stained paraffin sections of MTBs from a series of 17 rats aged 2-3, 6, 18, and 24 months. Between 2-3 and 24 months a 34% decrease in the mean number of MTB neurons was observed. Significant loss (p less than 0.05) was first evident in the early portion of the life span, between 2-3 and 6 months. In thionin-stained sections, there was no change with aging in the proportions of three MTB neuron types: principal cells (approximately 82%), elongate cells (approximately 15%), and stellate cells (approximately 3%). In young adult rats, 25-26% of all MTB neurons were associated with identifiable chalices of Held in protargol stained sections. This ratio did not vary significantly with aging. Age pigment accumulation in the MTB was examined in 2 micrometers Araldite sections stained with toluidine blue. Age pigment deposits were larger and more numerous in the MTBs of old animals, but not as extensive as has been described previously in many other parts of the nervous system. This study is the first to report neuron loss in an animal brainstem nucleus.