Prostanoids formed by cyclooxygenase play an important role in pain and the induction of inflammation. It is generally believed that COX-1 is constitutively expressed, whereas COX-2 is primarily inducible during inflammation. This study examined the in vivo selectivity of celecoxib, a COX-2 inhibitor, and evaluated whether estimates of selectivity that are based on in vitro and ex vivo analyses are reliable indicators of in vivo selectivity.