The importance of mother's milk as a source of docosahexaenoic acid (DHA) in Japanese monkey neonates was investigated. The DHA content in monkey colostrum total lipids was 2.2%, similar to or slightly higher than in humans. A comparison of the biosynthetic capacity of brain microsomes from monkeys of different age (up to 10 years) showed that chain elongation/desaturation of linolenic acid and eicosapentaenoic acid in neonates was significantly less pronounced than in adults. In particular, the formation of DHA, which is the product of delta 4 desaturase, was negligible. These results suggest that milk is an important source of DHA in Japanese monkey neonates.