1 ATP-sensitive K þ (K ATP ) channel activation is implicated in the vascular hyporeactivity occurring in septic shock. However, channel inhibition with the sulphonylurea receptor (SUR) antagonist, glibenclamide (Glib) fails to reverse lipopolysaccharide (LPS)-induced vascular hyporeactivity in vitro. We investigated whether inhibitors that act by binding to the K ATP channel pore could be effective. 2 Ring segments of endothelium-intact rat mesenteric artery were incubated with LPS in culture media for either 6 or 20 h before contractile responses to phenylephrine were assessed in the absence or presence of K ATP channel inhibitors. 3 The pore-forming subunit inhibitors barium chloride (BaCl 2 ; 300 mM) and PNU-37883A (1 mM) significantly reversed hyporeactivity at both time points, although less so at 20 h. In contrast, the SUR inhibitors, Glib (10 mM), tolbutamide (Tolb) (1 mM) and PNU-99963 (1 mM) were ineffective. In LPSincubated tissues, Glib and Tolb antagonised contractions to the thromboxane A2 mimetic, U46619 (9,11-dideoxy-9a, 11a-methanoepoxy prostaglandin F 2a ) (10 À7 M), whereas the pinacidil-derived inhibitor, PNU-99963, did not. 4 Contractions to 60 mM KCl were unaffected by LPS at 6 h, but were significantly depressed by LPS at 20 h, suggesting that K þ -channel-independent pathways contribute to hyporeactivity at the later time point. 5 The inducible nitric oxide synthase (iNOS) inhibitor, 1400 W (10 mM) and Tolb inhibited the production of nitrite induced by LPS, whereas BaCl 2 and PNU-37883A had no effect. 6 In conclusion, K ATP channels contribute to LPS-induced vascular hyporeactivity via the iNOS pathway in rat mesenteric artery. The effectiveness of pore inhibitors over SUR inhibitors of the K ATP channel suggests altered SUR function following LPS administration, which cannot be explained by thromboxane receptor inhibition.