Inhibition of cyclooxygenase (COX) activity decreases eicosanoid production and prevents lung cancer in animal models. Prostaglandin (PG) I₂ (PGI₂, prostacyclin) is a PGH₂ metabolite with anti-inflammatory, antiproliferative, and antimetastatic properties. The instability of PGI₂ has limited its evaluation in animal models of cancer. We hypothesized that pulmonary overexpression of prostacyclin synthase may prevent the development of murine lung tumors. Transgenic mice with selective pulmonary prostacyclin synthase overexpression were exposed to two distinct carcinogenesis protocols: an initiation/promotion model and a simple carcinogen model. The transgenic mice exhibited significantly reduced lung tumor multiplicity (tumor number) in proportion to transgene expression, a dose-response effect. Moreover, the highest expressing mice demonstrated reduced tumor incidence. To investigate the mechanism for protection, we evaluated PG levels and inflammatory responses. At the time of sacrifice following one carcinogenesis model, the transgenics exhibited only an increase in 6-keto-PGF_1α, not a decrease in PGE₂. Thus, elevated PGI₂ levels and not decreased PGE₂ levels appear to be necessary for the chemopreventive effects. When exposed to a single dose of butylated hydroxytoluene, transgenic mice exhibited a survival advantage; however, reduction in alveolar inflammatory response was not observed. These studies demonstrate that manipulation of PG metabolism downstream from COX produces even more profound lung cancer reduction than COX inhibition alone and could be the basis for new approaches to understanding the pathogenesis and prevention of lung cancer.