Nerve fibers containing dynorphin (Dyn) A(1-17)-like immunoreactivity were identified around cerebral arteries of guinea pig. The immunoreactive nerve fibers were richly distributed in anterior and middle cerebral arteries, but sparsely in posterior cerebral and basilar arteries. Histofluorescent study showed that large and small cerebral arteries were abundantly innervated by monoamine nerve fibers. Pretreatment with 6-hydroxydopamine or reserpine reduced the concentration of Dyn A in the wall of arteries by about 60% and 30%, respectively. These results demonstrated that there exist Dyn A immunoreactive nerve fibers in cerebral arteries and Dyn A may coexist with monoamine in perivascular nerve fibers.