Samples of blood were obtained from 52 primiparous breast-feeding women 4 days post partum. Thirty-six of the mothers were still breast-feeding 3-4 months later and had further blood samples taken. Somatostatin levels were analyzed by radioimmunoassay. A highly significant rank correlation (P = 0.0001) between average somatostatin levels on the two occasions was established, although somatostatin levels recorded 3-4 months post partum were significantly higher than those found 4 days post partum (P less than 0.01). Furthermore, somatostatin levels obtained 4 days and 3-4 months post partum were inversely related to the birth weight of their children (P = 0.006 and P = 0.03). The significant negative correlation between somatostatin levels recorded 4 days post partum and birth weight of the infants persisted only when non-smokers were investigated. A strong positive correlation between infant birth weight and weight of placenta was found (P = 0.0001) and a negative correlation (P = 0.04) between somatostatin levels and placental weight. A stepwise regression was performed to explain the importance of somatostatin levels in birth weight. Somatostatin levels and smoking had an almost equal influence on the variation in birth weight (approximately 10%). It is suggested that low maternal somatostatin levels are related to an efficient storage of nutrients in the fetoplacental unit, thereby leading to a high birth weight.