Summary:The role of the phosphodiesterase type IV iso zyme (PDE IV) in the regulation of cerebrovascular tone was investigated in the canine basilar artery in vitro and in vivo. The PDE isozymes extracted from the canine basilar artery were isolated by diethylaminoethanol (DEAE) Sepharose affinity chromatography and identified based on sensitivity to isozyme-selective PDE inhibitors. The family of phosphodiesterase (POE) isozymes is responsible for the intracellular hydrolysis of cAMP and cOMPo Some members of the seven POE gene families differ in their structure, substrate speci ficity, sensitivity to selective inhibitors, and tissue dis- Abbreviations used: DEAE, diethylaminoethanol; EC50s, con centrations producing 50% relaxation; EDT A, ethylenediaminetetra acetic acid; IC50s, concentrations producing 50% inhibition; PDE, phosphodiesterase; SAH, subarachnoid hemorrhage. In conclusion, PDE IV appears to be the predominant isozyme regulating vascular tone mediated by cAMP hy drolysis in cerebral vessels. In addition, vasorelaxation modulated by PDE IV is compromised in chronic cere bral vasospasm associated with subarachnoid hemorrhage.