ENDOCRINE and neuroendocrine systems have long held the fascination of gerontologists, since alterations in these integrative networks might underlie either the aging process itself or the clear-cut clinical manifestations of altered homeostasis in old age. With the exception of obvious changes in female reproductive endocrinology, most investigations of endocrine systems in man, including studies of pituitary, thyroid, parathyroid, pancreatic, and adrenal cortical function, have failed to show a major effect of aging on basal hormone levels and have displayed only modest changes in these systems as a result of stress (1). An exception to this generalization may be found in the age-related declines in renin and aldosterone (2, 3), changes that are probably secondary to decline in renal mass. During the last several years, considerable attention has been focused on the possibility that changes in neurotransmitter systems in the central nervous system may either underlie aging or its manifestations. Although a large number of studies in man and other mammals have identified changes (generally decreases) in the central nervous system content of various neurotransmitters with advancing age, the marked variability between specific brain regions and specific neurotransmitter systems and the interspecies differences have retarded development of a uniform view of changes in brain neurotransmitter function with age (4).