Taurine and other free amino acids have been determined in human milk of a number of other species. Taurine is the most abundant free amino acid in the milk of the gerbil, mouse, cat, dog and rhesus monkey. Taurine is the second most abundant amino acid in the milk of the rat, baboon, chimpanzee, sheep, Java monkey and man. Taurine is not a major constituent in the milk of the guinea pig, rabbit, cow and horse. The milk of each species has a characteristic free amino acid pattern which may be an indication of the relative nutritional importance of these compounds during early postnatal development.