The effects of different levels of dietary Mn and different models of 54Mn administration and the effect of sucrose and starch on absorption and excretion of the isotope were studied. Rats were fed diets containing between 1.3 and 82.4 mg Mn/kg for 7 or 14 d and administered 54Mn by gavage or a test meal containing 5 or 20 micrograms Mn. Additional rats for each dietary treatment received 54Mn by intramuscular or intraperitoneal injection. Amount of Mn in the oral dose did not affect 54Mn absorption, but increasing dietary Mn reduced Mn absorption and enhanced 54Mn excretion. Absorption of 54Mn by fasted, gavaged rats was four times higher than in unfasted gavaged rats or in fasted rats fed test meals. Orally administered 54Mn had a shorter biological half-life than injected 54Mn and tissue distribution of 54Mn differed in rats given 54Mn by different routes. Rats fed between 1.4 and 2.8 mg Mn/kg diet grew as well as or better than those fed amounts similar to the recommended level (50 mg/kg). Sucrose-fed rats absorbed more than 54Mn than starch-fed rats. Rats fed sucrose excreted injected 54Mn faster than rats fed starch. Concentrations of liver Mn in sucrose-fed rats were higher than in starch-fed rats. Our results indicate that both absorption and excretion are important in maintaining Mn homeostasis in rats.