The amount of lipofuscin in neurons of two brainstem nuclei, the inferior olivary nucleus (ION) and the lateral geniculate nucleus (LGN), was measured in the brains of 1 progeric, 2 phenylketonuric, 5 Down's syndrome and 13 normal individuals. Aggregations of lipofuscin granules in cresyl violet stained sections were examined with epi-fluorescent illumination, drawn with a camera lucida, and measured with a planimeter. The proportion of lipofuscin to cell size was a linear function of age over the age range examined, 14 to 92 years. There was no difference between progeric, phenylketonuric, Down's and normal brains in the amount of lipofuscin accumulated with age, nor was there a marked within-individual correlation between amounts of lipofuscin in the two nuclei when these amounts were subtracted from the regression line values to control for age. The proportion of lipofuscin to cell size was the same in neurons of the LGN showing transneuronal atrophy in response to right eye pathology as it was in non-atrophic neurons.