The contents of non protein nitrogen, urea, taurine and other free amino acids were determined in the milk of cynomolgus monkey (Macaca fascicularis). The milk samples were collected longitudinally from 12 healthy cynomolgus monkey mothers at 2 to 4, 14, 30, 60 and 90 days of lactation. The crude protein content of the milk attained to the level of mature milk after 14 days of lactation. The non protein nitrogen of the milk constituted 12 to 14% of the total nitrogen and the urea nitrogen 50 to 60 % of the nonprotein nitrogen. Glutamic acid, taurine, glutamine, aspartic acid, alanine and serine were abundant in milk. It was found that free amino acid patterns of cynomolgus monkey have close similarities to those of human milk. Ethanolamine, phosphoethanolamine and phosphoserine were observed in cynomolgus monkey milk like those in human milk. The blood samples from cyno-molgus monkey mothers and their own infants were collected at the same time of milking for free amino acid analysis. The concentrations of most free amino acids in the milk were lower than those in the mother serum, except for taurine, aspartic acid, glutamic acid and glutamine. The contents of ethanolamine, phosphoethanolamine and phosphoserine in the milk were higher than those in the mother serum. The urea levels in milk and mother serum were similar. The infant serum was higher than mother serum in taurine and glutamic acid, and lower in proline and alanine. Serum taurine concentrations of cynomolgus monkey infants were higher than those of human breast-fed infants at 30 days after birth.