We asked if down regulation of growth hormone (GH) signaling could block carcinogenesis in the Probasin/TAg rat, a model of aggressive prostate cancer. The Spontaneous Dwarf rat, which lacks GH due to a mutation (dr) in its GH gene, was crossed with the Probasin/TAg rat, which develops prostate carcinomas at 100% incidence by 15 weeks of age. Progeny were heterozygous for the TAg oncogene and homozygous for either the wild-type GH gene (TAg/Gh^+/+) or the dr mutation (TAg/Gh^dr/dr). Prostate tumor incidence and burden were significantly reduced, and tumor latency was delayed in TAg/Gh^dr/dr rats relative to TAg/Gh^+/+ controls. At 25 weeks of age, loss of GH resulted in a 20% and 80% decrease in the area of microinvasive carcinoma in the dorsal and lateral lobes, respectively. By 52 weeks of age, invasive prostate adenocarcinomas were observed in all TAg/Gh^+/+ rats, while the majority of TAg/Gh^dr/dr did not develop invasive tumors. Suppression of carcinogenesis could not be attributed to alterations in prostate expression of TAg or androgen receptor, nor changes in serum testosterone levels. As carcinogenesis progressed in TAg/Gh^+/+ rats, prostate GHR mRNA and protein expression increased significantly but prostate IGF-I receptor mRNA and protein levels dropped. Furthermore, serum IGF-I and prostate IGF-I levels did not change significantly over the course of carcinogenesis. These findings suggest that GH plays a dominant role in progression from latent to malignant prostate cancer driven by the powerful probasin/TAg fusion gene in rats and suggest that GH antagonists may be effective at treating human prostate cancer.