Pancreases from normoglycaemic neonates (less than 15 days), infants (6 months) and adults were examined using immunoperoxidase techniques. Sections taken from five regions of the gland were analysed by morphometry. The volume density of total endocrine tissue was found to be higher than previously reported: 15% in neonates, 6-7% in infants and 2-3% in adults. In neonates, many endocrine cells were located in small clusters, sometimes budding from ducts, and up to 15% were isolated in ducts and acini. Similar clusters were still present, though less frequent, in infants. The relative proportion of all cell types varied only slightly between the different regions, except in the posterior part of the head, which comprised 90% of all PP(polypeptide) cells. With age, the proportion of somatostatin cells decreased (from about 30% in neonates to about 10% in adults), that of insulin cells increased (50 to 70%) and that of glucagon cells remained stable (20%). In the posterior part of the head, the proportion of PP cells tended to be higher in adults than in neonates or infants. This study shows that both the endocrine cell populations and the proportion of endocrine tissue in the pancreas change markedly in early life. It also suggests that "nesidioblastosis" is a normal feature of the pancreas of normoglycaemic neonates and young infants.