The sympathetic innervation of the main ventral artery of the rat tail has been studied using electrophysiological, histochemical and biochemical techniques. Excitatory junction potentials were evoked in the smooth muscle cells of an isolated segment (1-2 cm long) of the proximal vessel by stimulating its proximal end with a suction electrode. The amplitude of these potentials decreased distally until they were undetectable 6-8 mm from the stimulating electrode. The density of noradrenergic terminals along the length of the vessel was examined using formaldehyde-induced fluorescence and assay of endogenous noradrenaline, before and after lesioning the perivascular nerve plexus by freezing, or dividing the dorsal and ventral collector nerves. The results show that sympathetic axons run along the artery to make functional contact over only a few mm. Most (about 80%) of the sympathetic supply reaches the vessel after branching from the ventral collector nerves; the rest arises via the dorsal collector nerves. The location of the cell bodies of origin of the axons in the collector nerves has been identified after retrograde labelling with horseradish peroxidase. Motor and sensory axons in both dorsal and ventral nerve trunks arose from segments S2 to Co3, while almost all sympathetic neurons (95-98%) projecting to the tail were located in paravertebral ganglia S1 to S4 and the coccygeal ganglion. These results provide the first description of the relationship of the sympathetic innervation of the rat tail to its motor and sensory supply.