The ontogenic patterns of somatostatin content and its binding sites were studied in small intestine in the developing rabbit from birth until the adult stage. A peak of somatostatin concentration was observed immediately after birth (day 0) and at day 10 in duodenum and jejunum, followed by a decrease at day 15 and a new and gradual augment thereafter. Ileal somatostatin concentrations decreased after birth up to day 15 and then increased progressively with age until the adult period. The somatostatin binding capacity in cytosol of the mucosal layer of duodenum, jejunum and ileum decreased from birth until 10 days and increased thereafter up to the adult period. However, no age differences in dissociation constants were observed. Interestingly, there was an apparent lack of high-affinity sites immediately after birth (day 0) whereas two classes of binding sites were seen thereafter. These results suggest that somatostatin may be involved in the functional and anatomical development of the small intestine during the neonatal period.