The quantitative trait locus (QTL) Edpm3 is one of a group of additively acting QTL responsible for the difference in estrogen-induced pituitary tumor growth between the tumor-susceptible F344 and tumor-resistant BN rat strains. The F344.BN-Edpm3BN rat strain was produced by moving the segment of rat Chr 3 between D3Mgh7 and D3Mgh13, which contains the Edpm3 QTL, from the BN strain into the F344 genetic background. In a previous study, we used this congenic line to find that the BN allele of the Edpm3 QTL reduces tissue mass and S-phase fraction in the estrogen-induced rat pituitary tumor. We now report on the use of this congenic line to investigate the linkage of Edpm3 to tumor angiogenesis. Contrary to expectation, the F344.BN-Edpm3BN strain has significantly greater angiogenic activity than does F344 in both treated and untreated rats. Microvessel count (MVC), perivascular space, and number of nonattached pericytes/pericapillary fibroblasts are all elevated in the pituitary by chronic estrogen treatment and their values are significantly greater in F344.BN-Edpm3BN than F344. Thus, although there is greater angiogenic activity in the pituitary of estrogen-treated F344.BN- £<ipm3BN rats, there is a deficiency in capillary maturation compared with F344.