Treatment of fetal rat hepatocytes with transforming growth factor beta (TGF‐ß) is followed by apoptotic cell death. Analysis of radical oxygen species (ROS) content and mitochondrial transmembrane potential (Δψ_m), using specific fluorescent probes in FACScan and confocal microscopy, showed that TGF‐ß mediates ROS production that precedes the loss of Δψ_m, the release of cytochrome c, and the activation of caspase 3. TGF‐ß induces a decrease in the protein and mRNA levels of bcl‐x_L, an antiapoptotic member of the Bcl‐2 family. In contrast, there is no change in the expression and/or translocation of Bax, a proapoptotic member of the same family. EGF maintains Bcl‐x_L, preventing Δψ_m collapse and release of cytochrome c. The presence of radical scavengers blocks the decrease in bcl‐x_L levels, Δψ_m collapse, cytochrome c release, and activation of caspase 3; in contrast, the presence of glutathione synthesis inhibitors such as BSO accentuated the effect. The incubation of fetal hepatocytes in the presence of ter‐butyl‐hydroperoxide alone produces a decrease in bcl‐x_L. These results indicate that during the apoptosis mediated by TGF‐ß in fetal hepatocytes, ROS may be responsible for the decrease in bcl‐x_L mRNA levels that precedes the loss of Δψ_m, the release of cytochrome c, and the activation of caspase 3, culminating in cell death.—Herrera, B., Alvarez, A. M., Sanchez, A., Fernandez, M., Roncero, C., Benito, M., Fabregat, I. Reactive oxygen species (ROS) mediates the mitochondrial‐dependent apoptosis induced by transforming growth factor ß in fetal hepatocytes. FASEB J. 15, 741‐751 (2001)