Thyroid papillary carcinomas are characterized by RET/PTC rearrangements that cause the tyrosine kinase domain of the RET receptor to fuse with N-terminal sequences encoded by heterologous genes. This results in the aberrant expression of a ligand-independent and constitutively active RET kinase. We analysed actin reorganization induced by the RET/PTC1 oncogene in PC Cl 3 rat thyroid epithelial cells. Differently from oncogenes Src, Ras and Raf, RET/PTC1 caused actin filaments to form prominent stress fibers. Moreover, stress fibers were identified in human thyroid papillary carcinoma cell lines harboring RET/PTC1 rearrangements but not in thyroid carcinoma cells negative for RET/PTC rearrangements. RET/MEN 2A, a constitutively active but unrearranged membrane-bound RET oncoprotein, did not induce stress fibers in PC Cl 3 cells. Induction of stress fibers by RET/PTC1 was restricted to thyroid cells; it did not occur in NIH3T3 fibroblasts or MCF7 mammary cells. RET/PTC1-mediated stress fiber formation depended on Rho but not Rac small GTPase activity. In addition, inhibition of Rho, but not of Rac, caused apoptosis of RET/PTC1-expressing thyroid cells. We conclude that Rho is implicated in the actin reorganization and cell survival mediated by the chimeric RET/PTC1 oncogene in thyroid epithelial cells, both phenotypes being cell type- and oncogene type-specific.