The gut endocrine gene, neurotensin (NT/N), is expressed in a strict temporally and spatially specific pattern. Utilizing a sensitive reverse transcription-polymerase chain reaction procedure, we analyzed foregut and midgut organs for NT/N expression, determined the earliest apparent time point that NT/N is expressed in the gastrointestinal tract and compared the temporal relationship of NT/N with other genes. NT/N expression was detected in the fetal and early postnatal stomach. In the small bowel, NT/N was expressed at the earliest fetal time (14 days) that the small bowel could be reliably delineated from other gut organs; in contrast, expression of sucrase-isomaltase was only apparent at 28 days after birth. NT/N was expressed in the fetal (12, 14 and 16 days) liver and then again on days 3 and 7 after birth; however, NT/N was detected only in the fetal (14 and 16 days) pancreas. Finally, NT/N expression was first detected at approximately 12 days gestation in the primitive foregut and midgut, thus occurring significantly earlier than actual intestinal cytodifferentiation. NT/N is widely expressed in the gastrointestinal tract during fetal development, suggesting the presence of a shared ancestral stem cell; NT/N expression is then restricted to the small bowel of the adult. The determination of the cellular factors regulating the expression of NT/N will provide a better understanding of the mechanisms responsible for the strict patterning of gene expression in the gastrointestinal tract and possibly gut differentiation.