Transformation of thyroid cells with either K-ras or H-ras viral oncogenes produces cell types with different phenotype and different response to the inhibition of the prenylation pathway by 3-hydroxy-3-methylglutaryl-CoA reductase or farnesyltransferase inhibitors. These inhibitors induce apoptosis in K-ras-transformed FRTL-5 cells (FRTL-5-K-Ras) whereas cell cycle arrest is induced in H-ras-transformed FRTL-5 (FRTL-5-H-Ras). In FRTL-5-K-Ras cells, the product of K-ras gene is implicated in the scavenging of reactive oxygen species (ROS) through the activation of extracellular-signal-regulated kinase (ERK)1/2 kinases. We observed that lovastatin blocked ras activation through inhibition of farnesylation and induced apoptosis, increasing ROS levels through inhibition of ERK1/2 signaling and Mn-SOD expression. Lovastatin-induced apoptosis was due to intracellular ROS increase since both, the antioxidant compound pyrrolidinedithiocarbamate or the SOD-mimetic compound, antagonized apoptosis. Moreover, both p38 mitogen-activated protein kinase and nuclear factor kappaB pathways, activated as a consequence of high ROS levels, are involved in the apoptotic effect, indicating that cell death induced by lovastatin was dependent on oxidative stress. Lovastatin antitumor efficacy in K-ras-dependent thyroid tumors was further confirmed in vivo, proposing a new therapeutic strategy for those tumor diseases that are sustained by an inappropriate K-ras expression.