IN 1931, Von Euler and Gaddum demonstrated the presence of a substance in extracts of brain and of intestine that causes contraction of isolated intestinal muscle in vitro and vasodilatation and hypotension in vivo1. These effects occur in the presence of atropine and hence are independent of acetylcholine. The active principle became known as substance P (P for preparation). Chang and Leeman recently isolated an un-decapeptide from bovine hypothalami and showed that it was substance P by multiple criteria2. The amino-acid sequence was determined3 and the peptide was synthesized by the solid-phase procedure4. The synthetic product was found to possess equipotent activity in multiple bioassays when compared to natural bovine substance P4. We now describe the development and application of a sensitive and specific radioimmunoassay for substance P.