Sprague-Dawley rats were chronically exposed to particulate Mn3O4 through two generations. At specific ages, observations were made of growth, tissue content, and distribution of Mn and Fe as affected by chronic exposure to Mn through an Fe-sufficient diet and an Fe-deficient diet. Chronic dietary Mn3O4 resulted in dose-related increases in Mn accumulation, and a concomitant Fe deficiency promoted Mn accumulation. In general, the addition of substantial amounts of Mn to either diet depressed tissue Fe levels.