The critical genes and products involved in estrogen-induced tumorigenesis of the pituitary gland were investigated in heterozygous transforming growth factor-beta (TGF-beta) receptor type II and p27 knockout mouse models. Tgfbr2(+/-), p27(+/-); Tgfbr2(+/-), and p27(+/-) mice and C57BL/6J wild-type mice received sc implantation of estrogen or placebo pellets for 16 or 25 wk, after which the mice were sacrificed and their pituitary glands removed for examination. The bromodeoxyuridine labeling indexes in tissue from both the anterior and intermediate pituitary lobes from p27 (+/-) and Tgfbr2(+/-); p27(+/-) mice were significantly higher than those from wild-type and Tgfbr2(+/-) mice after treatment with estrogen for 16 wk. Pituitary tumorigenesis was significantly accelerated in Tgfbr2(+/-), p27(+/-), and Tgfbr2(+/-); p27(+/-) mice compared with wild-type mice after treatment with estrogen for 16 wk. Pituitary tumorigenesis was not accelerated in Tgfbr2(+/-); p27(+/-) mice compared with Tgfbr2(+/-) or p27(+/-) mice. Expression of TGF-beta receptor type II mRNA was lower in the pituitary gland of Tgfbr2(+/-) mice than in wild-type mice before estrogen treatment and was significantly reduced after treatment. Pituitary tumorigenesis is accelerated in mice with severe TGF-beta resistance, and greatly accelerated in mice with TGF-beta resistance combined with decreased p27 expression compared with wild-type mice. Both the TGF-beta receptor type II gene and p27 gene and their products are involved in estrogen-induced tumorigenesis.