The cyclooxygenase-2 (COX-2) gene plays a role in a wide variety of normal physiologic pathways and is a major target of pharmacologic intervention in a large number of pathophysiologic contexts, including pain, fever, inflammation, and cancer. Expression of the COX-2 gene is induced in a wide range of cells, in response to an ever-increasing number of stimuli. The regulation of the COX-2 gene has been the subject of extensive study, using traditional transfection techniques with reporter gene constructs. Regulation of the COX-2 gene in living animals, however, requires sacrifice of the animal and in situ hybridization and/or immunohistochemical studies. We have utilized in vivo optical imaging technology with a cooled charged coupled device camera to image the expression of the firefly luciferase gene in tumor xenografts that are stably transfected with a chimeric gene containing the first kilobase of the murine COX-2 promoter. Induction of luciferase gene expression following systemic lipopolysaccharide/endotoxin administration can be robustly demonstrated; both a dose-response relationship and a time course for luciferase expression from the COX-2 promoter can be noninvasively analyzed in the tumor xenografts. These data suggest expression from the COX-2 promoter will be easily analyzed in transgenic mice, in knock-in mice, and in somatic cell and gene transfer experiments.