A longitudinal study of age-change in the hematocrit (Hct) was conducted with ad libitum fed and dietary restricted male F344 rats. A progressive fall in Hct occurred over the age range of 3 to 20 months. The magnitude of this decrease in Hct was similar in ad libitum fed and dietary restricted rats. Whether this age-associated decline in Hct continues after 20 months of age cannot be answered with certainty because of the occurrence of diseases at advanced ages which can mask or exacerbate the effects of the aging processes on the Hct. Studies of similar design with males and females of other strains of rats are needed to establish that a decreasing Hct is characteristic of aging in this species.