The metabolites of 2-(4-chlorobenzoylamino)-3-[2(1H)-quinolinon-4-yl]propionic acid (OPC-12759) (1), which has a potent antiulcer activity towards acetic acid-induced gastric ulcer, were synthesized to confirm their structures and to examine their antiulcer activity. The structures of the major metabolites (2-4) in the rat were identified by means of comparisons with the synthetic compounds. The antiulcer activity of the metabolites (2-4) was found to be lower than that of 1.