1. The objective of the present study was to investigate human omental arteries and veins with respect to: (i) the contractile effect of the thromboxane A2 analogue U46619, (ii) endothelium-dependency and mediators of the relaxing effect of substance P (SP) and acetylcholine (ACh). 2. Changes in isometric tension in response to administration of U46619, SP and ACh were measured in human isolated omental arteries and veins with and without endothelium. To investigate the mechanism of action of SP, the SP-induced relaxation was measured in the presence of indomethacin (cyclo-oxygenase inhibitor), NG-monomethyl-L-arginine (L-NMMA, nitric oxide-synthase inhibitor), KCl (inhibitor of endothelium-dependent hyperpolarization), tetraethylammonium (TEA; non-selective inhibitor of K(+)-channels, with some preference for the high conductance Ca(2+)-activated K(+)-channel, BKCa), glibenclamide (inhibitor of the ATP-sensitive K(+)-channel) and/or clotrimazole (inhibitor of the cytochrome P450-system and the intermediate conductance Ca(2+)-activated K(+)-channel, IKCa). 3. U46619 contracted both the artery and the vein segments. Endothelium removal did not alter the contraction. 4. ACh caused neither contraction nor relaxation in artery and vein segments precontracted with U46619. 5. In both artery and vein segments precontracted with U46619, SP produced endothelium-dependent relaxation. The relaxation was unaffected by indomethacin, but was incompletely reduced by L-NMMA and KCl respectively. The L-NMMA-resistent relaxation was abolished in the presence of KCl. 6. TEA inhibited the SP-induced relaxation in artery and vein segments both in the presence and absence of L-NMMA and indomethacin, while glibenclamide and clotrimazole had no effect. 7. In conclusion, the SP-induced relaxation in human omental arteries and veins seems to be mediated via NO and endothelium-dependent hyperpolarization. KATP and IKCa are probably not involved in the hyperpolarization, but activation of BKCa may contribute to the hyperpolarization. Prostanoid synthesis and the cytochrome P450-system are probably not involved in the SP-induced relaxation in this area.