There is apparently a large requirement for taurino (tau) in the neonatal period; for a number of species, such as nan and the cat, there is a dietary requirement because of limitRd ability to synthesize tau. We showed that oven in the rat, which can synthesize tau readily, a significant proportion of neonate brain tau originates from the milk. Therefore, we measured the free amino acid composition of milks from 15 species, including variation throughout lactation. Tau is a major constituent of most species milk: In rhesus monkey, dog, cat and gerbil milk tau is the free amino acid in highest concentration. In chimp, baboon and java monkey milk tau is exceeded only by glutamate; in sheep milk only by glycine and in rat milk only by ethanolamine. Species whose milk does not contain tau as a major constituent, i.e. cow, rabbit, guinea pig and horse, have glycine and/or glutamate as a major constituent. Horse milk was the only milk which contained neither tau nor glycine, the two amino acids usually involved in bile acid conjugation, as a major constituent. The milk tau concentration of man, cow, sheep and rat was highest in the first few days of lactation. The special importance of these results for feeding the human infant will be discussed in terms of the high concentration of tau in human milk and neonatal brain and the low intake of tau on cow milk and formulas now made from it.