In organs other than the brain, cell activation seems to increase "wear and tear," e.g., by increased free-radical formation, and so to cause an increased rate of aging. However, activation of nerve cells within the physiological range seems to lead to maintenance of neurons during aging and in Alzheimer's disease, possibly by preferentially stimulating the action of protective mechanisms such as DNA repair. This "use it or lose it" principle might explain why certain neurons degenerate in aging or Alzheimer's disease while others do not, and why recovery of various neuronal systems during aging has been obtained by restoration of the missing stimulus. Consequently, neuronal activation might provide a means of prolonging its optimal function for the full length of our natural life span.