Mesenteric arteries of the rat are surrounded by a plexus of primary afferent nerve terminals which contain both substance P (SP) and calcitonin gene-related peptide (CGRP). The ultrastructural arrangement of the innervation was studied in second-order branches of the rat mesenteric artery using immunohistochemical labelling with antibodies against SP. The structure and distribution of SP-immunoreactive (SPϩ) and SP-negative (SPϪ, i.e., virtually all noradrenergic) axons and their terminals within the adventitia of the artery have been determined. Sixteen percent of axons and 22% of varicosities in the perivascular plexus were SPϩ. Most of the SPϩ varicosities lay between 0.4 and 2 µm from the smooth muscle cells, whereas most SPϪ varicosities lay much closer to the vessel (i.e., Ͻ1 µm). SPϩ varicosities typically contained the same number and size of small synaptic vesicles and mitochondria as SPϪ varicosities, but there were more large dense-cored vesicles in the SPϩ varicosities. Unlike SPϪ varicosities, the peptidergic varicosities did not show clustering of synaptic vesicles toward one part of the axon membrane, and none of them formed junctions with the smooth muscle cells. Close relationships between SPϩ and SPϪ varicosities lacked any detectable structural specialization. The arrangement of SPϩ (primary afferent) terminals and their association with vascular smooth muscle cells indicates that peptide released from afferent terminals must diffuse further than noradrenaline from sympathetic terminals to reach the vascular smooth muscle.