THE EFFECTS of normal aging on cerebral circulatory physiology are of interest for several reasons. Many of the disease states in which cerebral vascular and metabolic functions are studied are commoner in middle-aged and elderly persons, making statistical comparisons with normal groups, presently comprised almost entirely of young subjects, inaccurate. In addition, the alterations which occur in cerebral circulatory functions with advancing age may provide additional clues to the understanding of many clinical problems of the aged, particularly those related to vascular disease of the brain; and such studies may provide the physiologist with further information concerning general patterns of circulation in subjects of middle and old age. The purpose of this report is to record our observations in the use of the nitrous oxide technique for measuring cerebral blood flow on 32 normal men between the ages of 38 and 79. It is planned to continue these studies so