Manganese (Mn) and magnesium (Mg) levels in hypothalamus, cerebellum, pons and medulla, striatum, midbrain, and cerebral cortex of control and Mn-treated (10 mg MnCl2.4H2O per ml of drinking water) rats during postnatal development were studied using instrumental neutron activation analysis. The age-dependent Mn accumulation showed regional variation: at day 5, this accumulation was most marked in striatum (12.05 micrograms/g wet weight) but least marked in cerebral cortex (0.85 micrograms/g wet weight). By day 10, pons and medulla, and hypothalamus were regions with, respectively, the highest (4.73 micrograms/g wet weight) and the lowest (0.52 micrograms/g wet weight) Mn levels. By contrast, brain regional Mn variations were less pronounced in weanling and adult rats. The age-dependent Mg accumulation showed regional variation at day 5, being most marked in pons and medulla (720 micrograms/g wet weight) and least marked in cerebral cortex (295 micrograms/g wet weight). Mg levels in all regions decreased after day 5; by day 120, only Mg level in cerebral cortex was lower than levels in other regions (the latter being very similar). In general, the age-related decreases in Mn and Mg levels paralleled the decreases in water content and increases in tissue weight, suggesting that the maturation of the blood-brain barrier may play important role(s) in brain Mn and Mg homeostasis. Chronic Mn-treatment from conception onwards altered the regional Mn and Mg distribution patterns during development. Our results are consistent with the hypothesis that chronic Mn toxicity exerts modulatory effects on brain regional metabolism and homeostasis of Mn and other metals during development.