The transgenic mouse strain FVB/N-Tg (MMTVneu)202Mul/J has been characterized as the “workhorse model of HER2 tumorigenesis in vivo.” These mice selectively express rat c-Neu/ErbB2 (HER2-neu) in their mammary glands via a mammary tumor virus (MMTV) promoter, which is responsive in cell cuture to glucocorticoids. We tested the hypothesis that exposure to environmental stressors in vivo would result in early onset of mammary tumors in this strain. Adult female mice, 4–5 weeks of age, were purchased; housed in an isolation chamber, and assigned to four experimental exposure groups: Restraint stress (90 min, 1x/wk, n = 24)[RS]; Social Disruption (cage mate switch 2x/wk, n = 23)[SD]; Restraint + Social Disruption (n = 22) and Home Cage controls (n = 20)[HC]. All mice were inspected (blind) for tumors weekly and humanely sacrificed one week post palpable tumor development. Consistent with the study hypothesis, the results indicated that mice chronically exposed to the two environmental stressors (RS& SD) developed primary tumors at a significantly (p