The perivascular autonomic nerves of the major blood vessels on the ventral surface of the brain were studied in the streptozotocin-induced diabetic rat, an animal model for juvenile onset of diabetes. Histochemical and immunohistochemical techniques were used to determine the pattern and density of perivascular nerves containing catecholamine, 5-hydroxytryptamine (5-HT), vasoactive intestinal polypeptide (VIP) and neuropeptide Y (NPY). A significant reduction in the density and/or fluorescence intensity of 5-HT-immunoreactive nerves was observed in the circle of Willis and its main arterial branches namely: basilar, superior cerebellar, internal carotid, posterior communicating, middle cerebral and anterior cerebral arteries, while a significant reduction of VIP-immunoreactive nerves was observed in the internal carotid, middle cerebral and anterior cerebral arteries, but not in the basilar, superior cerebellar and posterior communicating arteries 8 weeks after the onset of diabetes. However, no changes were observed in the density of NPY- and catecholamine-containing nerves. The results are discussed in relation to autonomic neuropathy of the cerebral blood vessels in diabetes.