1. We compared the effects of two novel tachykinin receptor antagonists, FK888 (selective at the tachykinin NK1 receptor) and FK224 (dual antagonist at NK1 and NK2 tachykinin receptors) on stimulus-evoked airway plasma exudation, bronchoconstriction and systemic hypotension in guinea-pigs in vivo. Plasma exudation was induced by substance P (SP), synthetic tachykinin receptor agonists, platelet activating factor (PAF), electrical stimulation of the cervical vagus nerves or by inhalation of cigarette smoke. Changes in airway tone and in carotid artery blood pressure (BP) were induced by synthetic tachykinin agonists, PAF and vagal stimulation. 2. Both FK224 and FK888 dose-dependently inhibited SP-induced plasma exudation in the lower trachea and main bronchi (ID50 values respectively of 1.1 and 0.1 mumol kg-1 in lower trachea, and of 0.5 and 0.1 mumol kg-1 in main bronchi) with complete inhibition at both airway levels at 10 mumol kg-1 for FK224 and at 2 mumol kg-1 for FK888. 3. The NK1-selective tachykinin receptor agonist, [Sar9,Met(O2)11]substance P ([Sar]SP), induced plasma exudation, a response which was blocked by both FK888 and FK224. The NK2-selective agonist, [beta-Ala8]neurokinin A-(4-10) ([beta-Ala]NKA), did not induce plasma exudation: neither FK888 nor FK224 affected this lack of response to [beta-Ala]NKA. 4. [beta-Ala]NKA induced bronchoconstriction, a response which was blocked by FK224 but which was completely unaffected by FK888. [Sar]SP induced a small but significant bronchoconstriction which was completely inhibited by both tachykinin antagonists. 5. In animals pretreated with capsaicin to deplete sensory neuropeptides, PAF induced both plasma exudation and bronchoconstriction. Neither response to PAF was inhibited by either FK888 or FK224.6. Both FK888 and FK224 inhibited plasma exudation induced by vagus nerve stimulation or by cigarette smoke, with FK888 more potent than FK224.7. FK224 inhibited non-cholinergic bronchoconstriction induced by vagal stimulation, whereas FK888,at doses inhibiting vagally-induced plasma exudation, did not.8. Decreases in BP induced by SP or [Sar]SP were blocked by both FK888 and FK224. In contrast,neither antagonist had any significant inhibitory effect on the decrease in BP induced by vagal stimulation (in the presence of atropine) or PAF. [beta-Ala]NKA did not decrease BP and neither tachykinin antagonist had any significant effect on this lack of response.9. We conclude that in guinea-pig airways, plasma leakage induced by endogenous tachykinins is mediated predominantly via NK1-receptors, whereas bronchoconstriction is mediated predominantly via NK2-receptors. In addition, SP-evoked decreases in BP are also mediated via NK1 receptors, whereas the contribution of endogenous tachykinins to vagally-induced decreases in BP appears to be minimal.Development of selective tachykinin receptor antagonists will be important in understanding the involvement of tachykinins in airway physiology and pathophysiology, whereas potent dual tachykinin receptor antagonists such as FK224 may have greater therapeutic potential in certain airway diseases in which tachykinins have been implicated in pathogenesis, including asthma and chronic bronchitis associated with cigarette smoking.