Tuberous sclerosis complex (TSC) is a genetic disorder caused by inactivating mutations in the TSC1 or TSC2 genes, which encode hamartin and tuberin, respectively. TSC is characterized by multiple tumors of the brain, kidney, heart, and skin. Tuberin and hamartin inhibit signaling by the mammalian target of rapamycin (mTOR) but there are limited studies of their involvement in other pathways controlling cell growth. Using ELT-3 cells, which are Eker rat–derived smooth muscle cells, we show that ELT-3 cells expressing tuberin (TSC2+/+) respond to platelet-derived growth factor (PDGF) stimulation by activating the classic mitogen-activated protein (MAP)/extracellular signal-regulated kinase kinase (MEK)-1–dependent phosphorylation of p42/44 MAP kinase (MAPK) with nuclear translocation of phosphorylated p42/44 MAPK. In contrast, in tuberin-deficient ELT-3 cells (TSC2−/−), PDGF stimulation results in MEK-1–independent p42/44 MAPK phosphorylation with reduced nuclear localization of phosphorylated p42/44 MAPK. Moreover, in TSC2−/− cells but not in TSC2+/+ cells, cellular growth and activation of p42/44 MAPK by PDGF requires the reactive oxygen species intermediate, superoxide anion (O₂^·−). Both baseline and PDGF-induced O₂^·− levels were significantly higher in TSC2−/− cells and were reduced by treatment with rapamycin and inhibitors of mitochondrial electron transport. Furthermore, the exogenous production of O₂^·− by the redox cycling compound menadione induced MEK-1–independent cellular growth and p42/44 MAPK phosphorylation in TSC2−/− cells but not in TSC2+/+ cells. Together, our data suggest that loss of tuberin, which causes mTOR activation, leads to a novel cellular growth-promoting pathway involving mitochondrial oxidant–dependent p42/44 MAPK activation and mitogenic growth responses to PDGF.