Prostaglandins (PGs) play important roles in the regulation of various gastric functions. In this study, the effects of various PGs on the growth of the human gastric carcinoma cell line KATO III were investigated. All the PGs tested inhibited KATO III cell growth with a relative potency order of PGE2 greater than PGE1 greater than 17S,20-dimethyl-6-oxo PGE1-methyl ester (ornoprostil) greater than PGF2 alpha. This inhibition was accompanied by an increase of cyclic adenosine monophosphate production. Furthermore, in the presence of guanosine triphosphate, these PGs stimulated adenylate cyclase activity in the plasma membrane of KATO III cells, followed by enhancement of membrane guanosine triphosphatase activity. The relative potencies of these PGs for increasing cyclic adenosine monophosphate levels, activating adenylate cyclase, and enhancing guanosine triphosphatase activity were all comparable to those for inhibiting cell growth. On the other hand, the proliferation of KATO III cells was also inhibited by forskolin as well as dibutyryl cyclic adenosine monophosphate, whereas none of the agents that did not increase cyclic adenosine monophosphate levels had any effect. These results suggest that PGs inhibit KATO III cell growth by stimulating cyclic adenosine monophosphate production via a guanosine triphosphate-dependent process, suggesting the involvement of guanosine triphosphate-binding stimulatory protein, probably coupled to PGE2 receptors, in the action of PGs.