1. The histamine-forming capacity, i.e. histidine decarboxylase activity, was studied in the acid-secreting mucosa of the human stomach employing a highly sensitive and specific isotope-dilution method. Activity was observed in each of six specimens. 2. The enzyme, histamine methyltransferase, which methylates ring nitrogen, was also present and the product, which is inactive as a gastric acid secretogogue, was observed. 3. The products of side-chain N -methylation, N -methylhistamine and N -dimethylhistamine, both of which are highly active in promoting acid secretion, could not be detected by biological and chromatographic techniques.