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Ultrastructural comparison hypothalamic neurons from the anterior, middle and posterior hypothalamus have been carried out both in young and aged Cynomologus monkeys. The overall ultrastructure of the hypothalamic neurons studied shows no fundamental difference. Neurons from the supraoptic, paraventricular and infundibular regions often exhibit a spiral arrangement of the smooth surfaced endoplasmic reticulum and the presence of neurosecretory granules (NSG). Lipofuscin is present both in the young and aged monkey. The progressive accumulation of lipofuscin and degenerative changes in the mitochondria are the main age related neurocytological changes. The rate of pigment accumulation however seems to differ in the various regions of the hypothalamus in the young and aged monkey. The young hypothalamus shows more pigment accumulation in the posterior part than in the anterior. In contrast, the aged hypothalamus revealed increased accumulation of pigment in the anterior part as compared to the posterior. The close topographical and morphological association of the pigment to the mitochondria and the pronounced degenerative age changes of the mitochondria, detrimental to oxidative and metabolic processes, suggest that lipofuscin is a stage in the life history of mitochondria and being a fundamental factor in aging. Noteworthy too is the increasing appearance of microglia cells or phagocytes filled with osmiophilic granules close to capillaries in the older age group of monkeys studied.