T o investigate roles in intestinal inflammation for the 2 cyclooxygenase (COX) isoforms, we determined susceptibility to spontaneous and induced acute colitis in mice lacking either the COX-1 or COX-2 isoform. We treated wild-type, COX-1^–/–, COX-2^–/–, and heterozygous mice with dextran sodium sulfate (DSS) to provoke acute colonic inflammation, and we quantified tissue damage, prostaglandin (PG) E₂, and interleukin-1β. No spontaneous gastrointestinal inflammation was detected in mice homozygous for either mutation, despite almost undetectable basal intestinal PGE₂ production in COX-1^–/– mice. Both COX-1^–/– and COX-2^–/– mice showed increased susceptibility to a low-dose of DSS that caused mild colonic epithelial injury in wild-type mice. COX-2^–/– mice were more susceptible than COX-1^–/– mice, and selective pharmacologic blockade of COX-2 potentiated injury in COX-1^–/– mice. At a high dose, DSS treatment was fatal to 50% of the animals in each mutant group, but all wild-type mice survived. DSS treatment increased PGE₂ intestinal secretion in all groups except COX-2^–/– mice. These results demonstrate that COX-1 and COX-2 share a crucial role in the defense of the intestinal mucosa (with inducible COX-2 being perhaps more active during inflammation) and that neither isoform is essential in maintaining mucosal homeostasis in the absence of injurious stimuli.