C-type natriuretic peptide (CNP), a member of the natriuretic peptide family which also includes ANP and BNP, is a putative endothelium-derived hyperpolarising factor (EDHF) in rat mesenteric resistance arteries (Chauhan, 2002) and the coronary vasculature as assessed in isolated perfused rat hearts (Brunner, 2001; Hobbs, 2004). However, there is also limited evidence to suggest that CNP also increases vascular resistance in dog mesenteric (Woods, 1999). We therefore investigated the effects of CNP in rat mesenteric and coronary arteries. Third-order (179 ± 3μm) and superior mesenteric arteries (576 ± 12μm) were dissected from humanely killed 12-week male Wistar rats and mounted on a small vessel wire myograph. All experiments used 300μmM L-NAME to block endothelial nitric oxide synthase. Results are shown as mean±S.E.M. and statistical analysis was performed with Student’s paired t test. In contrast to published work, we observed that CNP caused a further dose-dependent and endothelium-independent vasoconstriction when applied to U46619-preconstricted rat mesenteric resistance arteries (27 ± 4% contraction, n=6, P<0.001). This effect was observed in both Wistar and Sprague Dawley rats. The CNP-induced vasoconstriction was insensitive to blockers of PKC, PKA and tyrosine kinase but was sensitive to diltiazem (10μM), and was associated with a rise in the smooth muscle intracellular calcium concentration as measured using fura PE3. CNP up to 1μM had no effect on isolated mesenteric resistance arteries preconstricted with noradrenaline, and also had no effect on U46619-preconstricted rat coronary arteries. Conversely, CNP induced a vasodilatation in U46619-preconstricted rat superior mesenteric arteries (78 ± 3% relaxation, n=4, P<0.001) and had a similar effect in aorta. The relaxation was blocked when [K⁺] was raised to 25 mM (15 ± 14% contraction, n=4, P<0.001) and also by the combination of the EDHF inhibitors Ba²⁺ (50 μM) and ouabain (100μM), which block K_ir and the Na⁺, K⁺-ATPase, respectively. The relaxation was insensitive to the combination of TRAM-34 and apamin (2 ± 3% inhibition, n=4, n.s.), which block endothelial K_Ca channels. The NPR-C agonist, cANF (1μM), had no effect on either resistance (0 ± 2% n=4) or superior arteries (1 ± 1%, n=4) indicating that the action of CNP is not mediated through this receptor. These results show that CNP has opposing effects in the same arterial bed depending on the order/size of the artery in question. The relaxation observed in the larger arteries seems to occur via an EHDF mechanism, and the contraction in the resistance arteries appears to involve the activation of L-type Ca²⁺ channels. These effects were not mimicked by a selective NRP-C receptor agonist, suggesting that they could involve NPR-B receptors.