The metabolism and toxicity of lead, cadmium, mercury, and manganese in the postnatal period was studied in rats. Absorption, whole body retention, and organ distribution of 203Pb, 115mCd, 203Hg, and 54Mn were determined after oral and parenteral administration of these radioisotopes. The acute oral toxicity (LD50) was determined after a single application of metal chlorides. The results obtained in sucklings show a very high intestinal absorption of all metals which is partly attributed to milk diet; a higher whole body retention, higher blood levels and a much higher accumulation in the brain; and a higher oral toxicity. These results indicate age specific differences in the pharmacokinetics of metals in sucklings. It seems reasonable to consider the early neonatal age as a critical period for metal accumulation and therefore for metal toxicity. The results are interpreted on the basis of current concepts of developmental physiology and pharmacology and suggestions for future research trends are made.