Single nerve fibres innervating tooth pulp were isolated from filaments dissected from the inferior alveolar nerve in 17 anaesthetized cats. The fibres were studied to determine whether electrical stimulation of single units produced detectable changes in pulpal blood flow. Single pulpal nerve fibres were electrically stimulated at just above their thresholds and blood flow was recorded with a laser-Doppler flow meter from the pulp of the ipsilateral canine. The thresholds of single fibres in dissected filaments were determined either by recording antidromic action potentials from the tooth or by using a novel technique based on collision. Units that produced blood flow changes were further characterized by recording their response to hot, cold, osmotic and hydrostatic pressure, and mechanical stimulation of exposed dentine and to drying the dentine. Of 93 units isolated, 14 produced changes in pulpal blood flow when stimulated electrically at 1 or 10 Hz. All had conduction velocities (0.8–2.0 m s−1) in the C-fibre range. Ten produced vasodilatation and the remaining four, vasoconstriction. Five of the fibres that produced vasodilatation also responded to the hot stimulus, suggesting that they may form part of an axon reflex or similar mechanism. The four vasoconstrictor units did not respond to any form of stimulus other than electrical and were presumed to be sympathetic post-ganglionic fibres.