The effects of a combination of simvastatin, a cholesterol-lowering agent, and carmustine (BCNU; N,N'-bis(2-chloroethyl)-N-nitrosourea) on experimental C6 glioma were studied in vitro and in vivo. In vitro simvastatin and BCNU alone inhibited cell proliferation in a dose-dependent fashion. A subliminal concentration of simvastatin (0.1 microM) markedly and synergistically increased the BCNU toxicity to C6 glioma cells. The cytofluorimetric analysis of DNA from simvastatin-treated C6 glioma cells showed, besides the already described arrest in G1, an arrest/retardation in G2-M. Mitotic index from C6 cells incubated with simvastatin (10 microM) decreased by about 90%, indicating a specific C6 arrest/retardation in G2. The drug effects could be completely reversed by simvastatin withdrawal or mevalonate addition to the cultured cells. The combination of simvastatin and BCNU resulted predominantly from the profound retardation of cells in the G2-M compartment of the cell cycle. In vivo simvastatin (administered daily mixed with food) and BCNU (single i.p. injection), when given separately, caused a dose-dependent inhibition of labeling index in C6 glioma homografts (ID50, 61 mg/kg/day and 8.7 mg/kg, respectively). The combination of the lowest doses tested (simvastatin, 25 mg/kg/day and BCNU 0.3 mg/kg) resulted in a significant growth delay (compared to either drug alone) in C6 glioma (P < 0.05). There was no significant increase in toxicity as assessed by myelosuppression (WBC counts and bone marrow labeling index) and body weight. The results provide in vivo support for the combined use of simvastatin, a cholesterol-lowering agent, and BCNU in brain tumor treatment.