The existence of two distinct isoforms of cyclooxygenase (COX), which convert arachidonic acid to prostanoids, is now well established. COX-1, which is constitutively expressed in many tissues (including the gastrointestinal tract, platelets, and kidney) is responsible for producing prostanoids that regulate normal housekeeping or physiologic functions. In contrast, COX-2 is the inducible form responsible for the production of prostanoids in response to a variety of evoking stimuli in different tissues and for mediation of inflammation and pain in certain diseases. Since the identification of COX-2, a great deal of research has been devoted to elucidating and understanding its molecular and physiologic characteristics. As a result of research into the differences between COX-1 and COX-2, new insights into the role of each isoform in normal homeostasis and in their responses to exogenous stimuli have emerged. Besides its induction in cells at inflammatory sites, COX-2 is known to be induced in the kidney in response to sodium depletion or in hyperfiltration states; in postsynaptic excitatory neurons in the brain after electroconvulsive stimulation, in the ovary and uterus during ovulation and implantation; in intestinal epithelium after bacterial infection; as well as in colon adenoma and carcinoma cells. These findings, largely from animal studies, have suggested a broader spectrum of biologic activity of COX-2 and potential alterations of specific physiologic or protective mechanisms by inhibition of COX-2, as well as potential new clinical targets of therapy with COX-2 inhibitors. As COX-2 appears to play an important role in pathologic processes other than pain and inflammation, ongoing research is investigating the potential utility of COX-2 inhibitors in other conditions, such as colonic polyposis, colorectal cancer, and Alzheimer's disease.