In three groups of rats discharge activity was recorded (i) from the peripheral stump of the cut saphenous nerve (saphenous-receptor preparation); (ii) from the central stump of the cut L4 or L5 dorsal root (dorsal root preparation); or (iii) from the peripheral stump of the saphenous nerve segment cut at both ends (axon preparation) during slow intraarterial infusion of capsaicin (30-300 micrograms/kg/min for 5 min) into the carotid artery. Capsaicin produced excitation, i.e. an increase in frequency of action potentials in the same dose range (100-300 micrograms/kg/min) in both the saphenous-receptor and dorsal root preparations, while the axon preparations remained unresponsive. In the cat, close arterial injection of capsaicin (up to 20 micrograms) into a collateral branch of the saphenous artery failed to evoke discharges in the saphenous axon preparation, although similar injection of 4-aminopyridine (60 micrograms), a K+ channel blocking agent was readily effective. These results indicate that after systemic application of capsaicin the peripheral and central endings of primary afferent neurons are equally important sites for activation and are much more sensitive to capsaicin than the axons of the nerve trunk.