The present study was designed to test the hypothesis that endogenous prostaglandin E (PGE) promotes the development, growth and metastasis of spontaneous mammary tumors in C3H/HeJ female retired breeder mice. The effect of chronic oral indomethacin (indo) therapy starting at 6 months of age was tested on these parameters as well as on animal survival, in comparison with control mice placed on 0.2% ethanol in drinking water for up to 25 months of age. Indo treatment delayed the initial (up to 27 weeks) development of primary tumors by 11-12 weeks; however, the subsequent rate of tumor appearance was unaffected (totaling 82% in indo-treated vs. 90% in controls by 25 months of age). Spontaneous regression of primary tumors (26% in controls) increased 2-fold (53%) with indo therapy. While the apparent reduction in the growth rate of primary tumors and the overall prolongation of animal survival were not significant, the lifespan of mice bearing multiple tumors was significantly prolonged by therapy. There was also a 2-fold reduction in the incidence of lung metastases in mice bearing detectable primary tumors, and this was more pronounced during the earlier phase of tumor development. Positive immunostaining for cyclooxygenase-2 enzyme (indicative of the cellular source of PGE) was exhibited by tumor cells, stromal cells and macrophages within the primary tumors. Tumors in indo-treated mice exhibited histological evidence of increased differentiation (acinar architecture), significant tumor cell death, mononuclear cell infiltration and reduction in vascularity, indicating that the beneficial effects of indo were due to multiple mechanisms, including improved immune response and reduced angiogenesis.