The gastric and antiulcer effects of UP 5145-52, a new naphthyridinone derivative (1-phenyl-1,2-dihydro-2-oxo-1,8-naphthyridin-3-yl)-4-acetoxy butane, were studied in rats. UP 5145-52 dose dependently prevented the formation of gastric lesions induced by a necrotizing agent such as absolute ethanol, indomethacin or stress (ED50 values were 0.065, 1.26 and 0.72 mg/kg p.o., respectively). A beneficial action was also observed against indomethacin-induced intestinal lesions. UP 5145-52 inhibited total acid output in the pylorus-ligated rat (ED50 on total acid output was 0.16 mg/kg p.o.) and in the rat chronic gastric fistula model. The mechanism of antisecretory activity currently remains unknown as UP 5145-52 fails to bind to receptors involved in gastric secretion (H2 or muscarinic) and to inhibit H+/K+ adenosine triphosphatase. The ability of UP 5145-52 (1 mg/kg p.o.) to restore synthesis of gastric acid glycoproteins depressed by previous administration of absolute ethanol suggests that cytoprotection could play a role in the antiulcer efficacy of the drug.