Various approaches might be employed in an effort to increase efficacy of the chemotherapeutic treatment of cancer. Recently, various modulators of anticancer therapy effectiveness have been studied. Antiproliferative effects of peripheral benzodiazepine receptor (PBR) ligands might be exploited to enhance cytotoxic effect of a chemotherapeutic drug towards cancer cells. In this work, we sought to enhance cytotoxic effect of etoposide (VP-16) by a PBR ligand, diazepam (DZ) in U-87 MG human glioma cells. Cytotoxicity of VP-16, DZ and their combinations was assessed by using the microculture MTT assay. Cell survival, effective concentrations (EC) and the onset of cytotoxic effect were determined. After 72 h of cultivation, survival of U-87 MG cells was reduced to 57 +/- 7% in the presence of VP-16 at 12.5 microg/mL alone, whereas DZ at 10-4 mol/L alone caused 28 +/- 6% reduction in cell survival. Coincubation of VP-16 at 12.5 microg/mL with DZ at 10-4 mol/L led to a further decrease in cell survival to 45 +/- 6%. Furthermore, DZ at 10-4 mol/L significantly decreased effective concentrations, EC10, EC30 and EC50, of VP-16 and the dose-response curves were shifted to the left. Addition of DZ at 10-4 mol/L to VP-16 also facilitated the onset of its cytotoxic effect. The same decrease in survival was thus achieved approximately 30 h earlier in comparison with VP-16 alone. However, DZ at 10-9 mol/L failed both to exert any effect on glioma cells survival and enhance cytotoxic effect of VP-16. DZ at 10-4 mol/L was capable of both reducing U-87 MG glioma cells survival when applied alone and also enhancing the cytotoxic effect of VP-16. No such observation was made for the lower concentrations of DZ. Potential implementation of diazepam in the antiglioma/anticancer armamentarium awaits further experimentation but phase I and phase II clinical trials could be suggested.