To study the effects of azathioprine alone and combined azathioprine and methylprednisolone on chronic cerebral vasospasm. An animal model of "double subarachnoid hemorrhage" was established by two cisterna magna injections of non-heparinized autologous arterial blood (0.3 ml/kg). The preventive effect of azathioprine alone and its combination with methylprednisolone on chronic cerebral vasospasm was studied. By angiography of vertebrobasilar artery with a transfemoral catheter, assay of LPO (lipid peroxidation) was performed on BA wall. Chronic cerebral vasospasm following subarachnoid hemorrhage was noted and the drugs' effect on chronic cerebral vasospasm also observed. Seven days after subarachniod hemorrhage, the basilar arterial (BA) diameter was 87% +/- 26% of the original BA diameter, and LPO content of BA wall was 0.03 +/- 0.01 nmol/mg in azathioprine group and 93% +/- 20% (BA diameter) and 0.02 +/- 0.01 nmol/L (LPO content) in azathioprine plus methylprednisolone group. When the BA diameter was 53% +/- 19%, the LPO content was 0.11 +/- 0.05 nmol/mg in the control group. Pathological examination showed that azathioprine alone and its combination with methylprednisolone obviously reduced the damage to the BA wall. Azathioprine has preventive effect on chronic cerebral vasospasm, supresses the production of free radicals and reduces damage to the BA wall following subarachnoid hemorrhage. Combined azathioprine methylprednisolone has produces better effects when only lower dosage is used.