A conventional balance study with growing rats was conducted to evaluate experimental conditions for determining endogenous fecal manganese (Mn) excretion and, hence, true Mn absorption by the isotope-dilution technique. Thirty-four rats, with a mean initial live weight of 60 g, allotted to three groups of 8 animals and one group of 10 animals, were injected intramuscularly with a(54)Mn tracer dose and sacrificed after 4, 8, 12, and 16 d, respectively.In liver and serum, the specific radioactivity of Mn was the lowest among the tissues analyzed and its exponential rate of decrease over the period of d 4-16 was the highest. During the 8-d period, from d 9-16, apparent Mn absorption averaged 14.1% of intake (128.5 μg Mn/d). Assuming that the specific activity of Mn in liver of d 11 or, alternatively, in serum of d 16, was on the average representative of that of endogenous Mn in feces of d 9-16, it was computed that Mn of endogenous origin accounted for 9.0 and 9.3% of the total fecal excretion of the metal, and that true absorption amounted to 21.9 and 22.1% of Mn intake, respectively.