The interactive effects of manganese and iron on true absorption and endogenous losses of manganese were investigated by feeding rats three levels of manganese (0.9, 48 or 188 micrograms Mn/g diet) and two levels of iron (19 or 276 micrograms Fe/g diet) for 7 wk. After 45 d, half of the rats were fed 54Mn and half were injected intraportally with 54Mn complexed to albumin. The relative distribution of 54Mn in tissues was generally similar for rats when 54Mn was administered in these two ways. Manganese-deficient animals retained more of the isotope, had both higher apparent and higher true absorption of manganese, had a greater proportion of 54Mn in their livers and had a lower proportion of 54Mn in their muscles compared with animals fed adequate or high levels of manganese. High iron intake inhibited manganese true absorption, reduced tissue manganese concentrations and inhibited heart manganese-dependent superoxide dismutase activity. However, the greatest effect of dietary iron was on mucosal cell manganese concentrations. Endogenous losses of manganese were approximately 8% of the amount of manganese actually absorbed regardless of intake. Thus, control of absorption in the gut seems to be the major way that manganese homeostasis is maintained. Furthermore, iron seems to be depressing manganese absorption by inhibiting manganese uptake into the mucosal cells.