The numbers of ramified microglia stained by mouse monoclonal antibodies (OX-6 and X-42) in various regions of the brain were compared in the young (8 weeks) and aged (12 months) rats. OX-6-positive microglia increased in number with ageing, but OX-42-positive microglia did not. In both young and aged rats, the number of OX-6-positive microglia in white matter was higher than that in grey matter, but the number of OX-42-positive microglia in white matter was the same as that in grey matter. OX-42 stains microglia in both activated and resting states, whereas OX-6 stains mainly microglia in the activated state. Our findings suggest that microglia become activated with age-related changes, especially in white matter.