The concentrations of substance P-like immunoreactivity (SPLI) were measured in mesenteric nerves and vessels of control rats and rats treated with capsaicin during the second day of life. The results showed that there was a significantly higher concentration of SPLI in the jejunum than in the terminal ileum, and that there was a loss of SPLI from these tissues following neonatal treatment with capsaicin. The results suggest that the substance P-containing afferent innervation of the upper small intestine is greater than in the terminal ileum. The significance of these results to the Intensity or Summation hypothesis of visceral pain, and to the thresholds of visceral reflexes from different areas of the small intestine is discussed.

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