Sodium content was analysed from 360 breast milk samples of 45 mothers who delivered at term and from 206 samples of 22 mothers who delivered preterm in the first 4 weeks postpartum. The sodium content was consistently higher in the milk from mothers of preterm infants. In both groups, the sodium content of the breast milk decreased rapidly for the first 3 days, and then more slowly. After the first week, the daily variation of sodium concentration in the breast milk was minimal. There was no significant statistical difference in the sodium content of breast milk before compared with after feeding nor was there any difference in sodium content from either breast.