We investigated the effects of the electrical stimulation of a unilateral cervical vagal nerve on the blood flow in the trachea using laser Doppler flowmetry in urethane anesthetized Wistar King rats. Stimulation for 30 s at 1, 2, 5, 10, 20 or 50 Hz with 10 V intensity caused an increase in tracheal blood flow (TBF) in a frequency-dependent manner; the effects were most dominant with the 10-Hz stimulation among the six frequencies used. The increased responses of TBF with the muscarinic receptor antagonist atropine (1.0 mg/kg, i.v.) were significantly reduced when compared with those without atropine at 5 Hz stimulation (123.3 +/- 11.9% vs. 180.1 +/- 24.5%). This shows the existence of vasodilation due to a cholinergic mechanism. The increased responses of TBF after the ganglion blocking agent hexamethonium (20 mg/kg) i.v. administration were significantly reduced when compared with those without hexamethonium at 1, 2 Hz stimulation (1 Hz: 18.9 +/- 2.7% vs. 35.4 +/- 4.7%, 2 Hz: 40.5 +/- 8.9% vs. 58.8 +/- 6.7%); this shows the existence of vasodilation due to a non-cholinergic parasympathetic efferent mechanism which itself appears to be due to the release of neuropeptides such as VIP and PHI. The increased responses after hexamethonium administration were augmented probably because of the enhanced release of other neuropeptides like SP and CGRP especially at 10 Hz and 20 Hz stimulation. These findings suggest that the mechanism of vasodilation by the activity in the vagal fibers in the trachea of the rat has cholinergic and non-cholinergic efferent components and a non-cholinergic afferent component. In rats, the afferent component may play an important role in controlling tracheal vascular changes.