All adult animals and humans tested up to this time have controlled their tissue manganese concentrations by controlling primarily the rate of the metal's excretion. In sharp contrast, neonatal mice did not excrete manganese for the first 17-18 days of life, although absorption of the natural 55Mn as well as distribution, tissue accumulation, and mitochondrial accumulation of the radioactive 54Mn were vigorous. This suggested as initially avid accumulation of this essential micronutrient, supplied in scarce traces in mouse milk (54 ng/ml) by mothers consuming very much higher dietary concentrations (55,000 ng/g). The tissue accumulation was demonstrated analytically and was particularly impressive in the brain, which can be susceptible to both manganese poisoning and deficiency.