A potent in vitro inhibitor of lipid peroxidation, U78517F, has been employed to assess its influence on C6 astrocytoma growth in monolayer culture and tumor growth and protein extravasation in the rat C6 astrocytoma spheroid implantation model. Results demonstrate that 1 μM concentration of U78517F inhibits cell division, but is not tumoricidal to C6 astrocytoma cells in monolayer culture. Concentrations of 5 μM and above significantly decreased astrocytoma cell viability. Following spheroid implantation, rats were treated with one of these U785I7F regimes: 12 mg/kg/day for 13 days post-implantation, 4 mg/kg/day for 13 days post-implantation or 12 mg/kg/day commencing seven days post-implantation until 13 days post-implantation. Tumor wet and dry weights were lower in all treatment groups, but these decreases were not statistically significant. Protein extravasation as measured by Evans blue extravasation was not significantly reduced by any treatment regime used. It is concluded that U78517F inhibits C6 astrocytoma growth in monolayer culture at and above 5 μM concentrations, but the treatment regimes utilized did not significantly decrease tumor growth or permeability in the C6 astrocytoma spheroid implantation model.