To define the relationship between the immunologic reaction and the pathogenesis of cerebral vasospasm (VS) following experimental subarachnoid hemorrhage (SAH), we examined the effect of a cell mediated immunosuppressive agent, FK-506, isolated from Streptomyces tsukubaensis, by using the canine SAH model. There was a significant vasoconstriction in the basilar artery in the control group after SAH. This constriction, however was not successfully prevented by FK-506 or combination of FK-506 and steroid, since there was no significant difference in the vessel caliber size among these groups. The pathologic approach, accompanied by immunohistochemistry, could not discriminate the differences in the nature of the lesion between the untreated group and FK-506 treated groups, except for slight lymphocytic infiltrations present around the basilar artery of untreated group. Histopathologically, inflammatory reactions consisting of neutrophils, that were not suppressed by FK-506 treatment, were clearly seen around the spastic vessels in the subarachnoid space. Furthermore, several constrictive changes or degenerative alterations were also observed in the spastic vascular wall. Immunohistochemically, the deposition of IgG, IgM and C₃ was present in the intima and the luminal side of the smooth muscle layer, and capillary vessels of the brain stem. It is considered that this deposition was caused by increased vascular permeability in VS. On the basis of the above findings that the cell mediated immunosuppressive agent, FK-506 failed to prevent vasoconstriction or pathologic lesions but lymphocytic infiltrations, it is considered that the cell mediated immunopathogenesis may play little role in producing VS following SAH.