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The distribution of regulatory peptides was studied in the separated epithelium, lamina propria, submucosa and muscularis externa of the human jejunum. Gastrin, secretin, gastric inhibitory polypeptide, enteroglucagon and neurotensin immunoreactivity were almost confined to the endocrine cell-containing mucosal epithelium ( > 98% of the total content), only minor amounts of motilin being detected in non-epithelial layers (3.6 ± 0.7%, mean ± SEM, n = 7). Conversely, vasoactive intestinal polypeptide, substance P and mammalian bombesin were virtually limited to non-epithelial layers ( > 99%). Only somatostatin was found in all layers (44 ± 6.7% in the epithelium, 34 ± 5.2% in the lamina propria, 13 ± 2.9% in the submucosa, and 7.9 ± 2.8% in the muscularis). Substance P was found in higher concentrations in the mucosa, compared to submucosa and muscle (56 ± 10, 30 ± 4.0 and 29 ± 4.0 pmol/g, respectively), while vasoactive intestinal polypeptide was more abundant in the muscle (411 ± 52 pmol/g) compared to mucosa and submucosa (228 ± 64 and 219 ± 31 pmol/g, respectively). Only low levels of mammalian bombesin were measured, mainly in the muscle (6.9 ± 1.5 pmol/g, or 89 ± 3.6% of total content).