Carcinogenesis by oncogenic Ras and Her-2 involves enhanced proliferation of epithelial cells in vivo. However, hyperproliferation induced by these oncogenes, or their downstream pathways in vitro has mainly been studied in cultured, fibroblastic cell lines. Here, we demonstrate that oncogenic Ha-Ras or constitutively active Her-2 cause increased proliferation and cyclin D1 upregulation in fully polarized, mammary epithelial cells (EpH4), if cultivated as organotypic structures in three-dimensional collagen/matrigel matrices. Under standard culture conditions, however, these oncogenes failed to induce hyperproliferation. Using both specific low molecular weight inhibitors and Ras-effector-specific mutants, we dissected signaling pathways downstream of oncogenic Ras (PI3K, Mek1/MAPK) with respect to (i) hyperproliferation in collagen gels and tumorigenesis in mice and (ii) epithelial/mesenchymal transition (EMT). We show that the Ras-activated PI3K pathway is required to induce rapid tumor growth and enhanced proliferation of EpH4 cells in collagen gels, but fails to cause EMT in vitro and in vivo. On the other hand, Ras-dependent activation of the Mek1/MAPK pathway in EpH4 cells (previously shown to cause EMT and metastasis) did not induce hyperproliferation in collagen gels and caused only slow tumor growth. Our data thus indicate that Ras-dependent signaling through the PI3K- and MAPK pathways fulfil distinct, but complementary functions during carcinogenesis.