The endocrine gastro-entero-pancreatic (GEP) system of 10 human embryos was studied with special reference to neurotensin-immunoreactive cells. These cells are first present in the ileal and jejunal mucosa of 12 to 13 week old embryos. Thereafter the neurotensin-immunoreactive cells are found regularly in these segments of the gut with an increasing number towards the terminal ileum. At about the twentieth week of gestation, the neurotensin cells are detected also in the lower duodenum, i.e. the distribution pattern is more extensive in this age than in younger embryos or in adults.