1. In the presence of atropine and guanethidine (3 mumol/l each), electrical field stimulation (1-20 Hz) produced frequency-dependent relaxations of the histamine- (3 mumol/l) induced vascular tone in isolated rings from the guinea-pig pulmonary artery. The electrically-evoked relaxations were abolished by tetrodotoxin (1 mumol/l). The amplitude of these nerve-mediated, non-adrenergic non-cholinergic (NANC) relaxations was unaffected by removal of the vascular endothelium produced through rubbing of the internal surface. 2. Capsaicin (1 mumol/l) produced a prompt and sustained relaxation of the histamine-induced tone which was unaffected by removal of the endothelium. A second application of capsaicin 60-120 min later had no further relaxant effect, indicating desensitization. After in vitro capsaicin desensitization, the electrically-evoked NANC relaxations were abolished, both in the presence or absence of the vascular endothelium. 3. Substance P evoked a prompt and transient relaxation in precontracted arterial rings with intact endothelium and a transient small contraction in rings in which the endothelium had been mechanically removed. The selective NK-1 receptor agonist, [Pro9]-substance P sulfone closely mimicked the relaxation produced by substance P while the selective NK-2 or NK-3 receptor agonists had no relaxant effect. Tachyphylaxis to substance P did not modify the amplitude of the capsaicin-induced relaxation. 4. Human alpha calcitonin gene-related peptide (CGRP) produced a prompt and sustained relaxation both in the presence and absence of the vascular endothelium. 5. Ruthenium red (10 mumol/l) blocked the relaxation to capsaicin while leaving unaffected the relaxation to electrical field stimulation or CGRP (0.1 mumol/l).(ABSTRACT TRUNCATED AT 250 WORDS)