Cyclosporin A (CsA) is a clinically important immunosuppressive drug widely used to prevent graft rejection following organ or bone marrow transplantation. Although there are reports of serious neurologic alterations associated with the use of the drug, the precise mechanism of its action on the CNS still remains unknown. We studied the effects of CsA on the growth of C6 glioma cells. We found that CsA inhibits the growth of C6 glioma cells in a dose-dependent manner and induces morphological changes such as shrinkage of the cell body and loss of extensions followed by cell death. The analysis of DNA from CsA-treated cells revealed a ladder-like pattern of fragmented DNA. Acridine orange staining showed the occurrence of apoptotic changes in nuclear morphology. Apoptotic morphological alterations were prevented by the treatment with cycloheximide. Altogether, our findings suggest that the CsA-induced cell death of C6 glioma cells bears all the features characteristic of programmed cell death. We also observed a significant increase in the DNA-binding activity of AP-1 during CsA-induced apoptosis. The AP-1 induction preceded the appearance of apoptotic, morphological changes and was accounted for by an increase in the expression of c-Jun protein. The occurrence of increased levels of AP-1 complex and c-Jun protein during CsA-induced programmed cell death suggests its involvement in the induction of apoptosis.