1. Exogenous vasoactive intestinal polypeptide (VIP) infused into the pulmonary artery of isolated and ventilated lungs of guinea-pigs decreased, in a dose-dependent fashion (1.0-10.0 nmol), airway resistance and thromboxane B2 (TXB2, the stable hydrolysis product of TXA2) release in the perfusion medium. Prostacyclin (PGI2) synthesis, as reflected by the release of its stable hydrolysis product 6-oxo-PGF1 alpha, was unaffected. Pretreatment with the 5-lipoxygenase inhibitor BWA4c (3.5 x 10(-5) M) did not modify the bronchodilatory effect of VIP or its inhibitory action on TXB2 release. 2. Basal release of immunoreactive VIP from perfused lungs decreased from an initial value of 0.96 +/- 0.10 ng min-1 (mean +/- s.e.mean) in the first 2 min to an average of 0.58 +/- 0.10 ng min-1 in the following 15-20 min. 3. Antigen challenge with ovalbumin (0.1%) in sensitized lungs caused an anaphylactic reaction in 45% of tested lungs, concomitant with a 5 fold increase in both VIP and TXB2 release. Tetrodotoxin pretreatment (10(-6) M) reduced basal VIP release by > 80% and abolished the VIP increase observed during anaphylaxis, without modifying TXB2 release or the bronchoconstrictor response. 4. Indomethacin (10(-6) M) inhibited TXB2 synthesis and release by > 90%, delayed the bronchoconstrictor response and blunted the increased VIP release during lung anaphylaxis, without influencing basal VIP release. 5. The 5-lipoxygenase inhibitor BWA4c (3.5 x 10(-5) M) blunted the increase of TXB2 and VIP release from guinea-pig lung and attenuated the bronchoconstrictor response following ovalbumin challenge. 6. The administration of exogenous VIP as a continuous infusion (10-8 M) attenuated the bronchoconstriction and the release of cyclo-oxygenase metabolites following antigen challenge.7. Acetylcholine (10-6-l0-5 M) infused into the pulmonary artery induced a dose-dependent bronchoconstriction not associated with enhanced VIP or TXB2 release.8. The TXA2 mimetic U-46619 (0.1-1.0 nmol) caused dose-dependent increases in airway resistance,concomitant with an up to 10 fold increase in VIP release. VIP inhibited arachidonate-induced in vitro aggregation of washed rabbit platelets in a dose-dependent manner over a dose range 10-8 10-6 M.Despite the antiaggregatory effect of VIP, TXB2 and PGE2 synthesis was reduced only to a minor extent,and there was no redirection of arachidonate metabolism from TXA2 to PGE2, indicating that VIP does not act as a TX synthase inhibitor in vitro.9. We conclude that VIP may play a role in regulating bronchial smooth muscle reactivity in lung anaphylaxis by inhibiting the synthesis and release of TXA2, a potent vasoactive and bronchoconstrictor agent. TXA2, on the other hand, strongly enhances neuronal VIP release.