We have previously reported that transforming growth factor-β1 (TGF-β1) produces a 'prooxidant' effect on cultured bovine pulmonary artery endothelial cells (BPAEC) [S. K. Das and B. L. Fanburg. Am. J. Physiol. 261 (Lung Cell. Mol. Physiol. 5): L249-L254, 1991]. This effect was found to be associated with a lowering of total cellular GSH (A. C. White, S. K. Das, and B. L. Fanburg. Am. J. Respir. Cell Mol. Biol. 6: 364-368, 1992). In this study, we demonstrate a twofold increase in the rate of extracellular H2O2 release from BPAEC after a 72-h exposure to TGF-β1 (2 ng/ml, added at times 0 and 48 h). Increasing and decreasing the levels of cellular GSH with diethylmaleate (DEM, 0.05 mM) and buthionine sulfoximine (BSO, 0.01 mM), respectively, did not affect the rate of TGF-β1-induced increase in H2O2 release when compared with the individual effects of these reagents on control cells. The addition of BSO (0.01 mM) to control cells failed to demonstrate an increase in the rate of H2O2 release despite a more profound decrease in cellular GSH by these cells than detected in cells treated with TGF-β1 alone. Moreover, a single dose of TGF-β1 (2 ng/ml) induced a 63-85% increase in the rate of H2O2 release within 16 h of exposure, well before the previously demonstrated lowering of cellular GSH. These results indicate that the increase in H2O2 production by TGF-β1-stimulated BPAEC is associated with, but does not appear to be the result of, a lowering of cellular GSH. This study further suggests that the TGF-β1-induced H2O2 production occurs at a site inaccessible to detoxification by GSH.