The flux of glucose in the pathways of acetyl group formation and disposal and the activities of a range of enzymes related to these processes have been measured in the brains of rats aged between 1 day and 24 months. The pattern of enzyme change is such that those systems involved in hydrogen transfer appear to increase disproportionately, relative to the glycolytic flux in the aged brain. It is suggested that these increases are an essential corollary to the need for a maintained glycolytic flux in a tissue dependent upon glucose as a fuel and one in which alternative routes of NADH oxidation diminish with age.