The antropyloric mucosa of rat, rabbit, cat, pig and man was investigated using certain modifications of the original Falck-Hillarp procedure. In rabbit, cat, pig and man, combined formaldehyde-ozone treatment induced intense fluorescence in a large number of epithelial cells. By subsequent treatment of the sections for immunohistochemical detection of gastrin (immunofluorescence or immunoperoxidase labeling), it could be shown that these cells were identical with the gastrin cells. The gastrin cells of cat and man could be visualized also by treatment with formaldehyde-HCl, a feature not shared with the gastrin cells of rabbit and pig. The gastrin cells in the rat antrum gave no fluorescence with either formaldehyde-ozone or formaldehyde-HCl. From histochemical model experiments and cytospectrofluorometric analyses it appears probable that the formaldehyde-ozone-induced fluorescence reflects the presence of a peptide with NH2-terminal tryptophan. The chemical background of the fluorescence induced by formaldehyde-HCl treatment is as yet obscure. The gastrin cells of all animals studied stained with silver in the Grimelius method. Argentaffin and diazo staining as well as silver staining according to Hellerström and Hellman were carried out on sections from human antrum only. The gastrin cells were negative with these stains, but they stained with lead-hematoxylin. These properties support the view that the gastrin cells are identical with the so-called G cells.