The expression of COX-2 is up-regulated in the rat stomach after administration of indomethacin, and the inhibition of this enzyme may be a key to NSAID-induced gastric damage. The present study investigated the mechanism for COX-2 expression induced in the rat stomach by indomethacin, in relation with the ulcerogenic processes. The animals were given indomethacin or SC-560 p.o., and the gastric mucosa was examined 8 hr later. Indomethacin decreased the mucosal PGE2 content and produced gross damage with gastric hypermotility and the expression of COX-2 mRNA in the mucosa. Although SC-560 did not produce damage, this agent caused a decrease in the PGE2 content and an increase in gastric motility as well as the up-regulation of COX-2 expression, and provoked damage in the presence of rofecoxib. Gastric lesions induced by indomethacin were prevented by both atropine (even in the presence of exogenous HCl) and omeprazole, although the hypermotility response was inhibited only by atropine. The COX-2 expression induced by indomethacin or SC-560 was inhibited by atropine, even in the presence of exogenous HCl, while omeprazole had no effect. The mucosal PGE2 content was decreased by SC-560 at 2 hr but recovered 8 hr later, and this recovery of PGE2 was attenuated by both atropine and rofecoxib but not omeprazole. These results suggested that the COX-2 expression in the stomach following treatment with indomethacin is functionally associated with gastric hypermotility response induced by COX-1 inhibition. Luminal acid does not play a role in the up-regulation of COX-2 expression in the stomach following administration of indomethacin.