BACKGROUND AND PURPOSE We evaluated the role(s) of monoamine oxidase (MAO)-mediated H O generation on 5-hydroxytryptamine (5-HT)-induced tension development of isolated basilar artery of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. EXPERIMENTAL APPROACH Basilar artery (endothelium-denuded) was isolated for tension measurement and Western blots. Enzymically dissociated single myocytes from basilar arteries were used for patch-clamp electrophysiological and confocal microscopic studies. KEY RESULTS Under resting tension, 5-HT elicited a concentration-dependent tension development with a greater sensitivity (with unchanged maximum tension development) in SHR compared with WKY (EC : 28.4 ± 4.1-nM vs. 98.2 ± 9.4-nM). The exaggerated component of 5-HT-induced tension development in SHR was eradicated by polyethylene glycol-catalase, clorgyline and citalopram whereas exogenously applied H O enhanced the 5-HT-elicited tension development in WKY. A greater protein expression of MAO-A was detected in basilar arteries from SHR than in those from WKY. In single myocytes and the entire basilar artery, 5-HT generated (clorgyline-sensitive) a greater amount of H O in SHR compared with WKY. Whole-cell iberiotoxin-sensitive Ca -activated K (BK ) amplitude measured in myocytes of SHR was approximately threefold greater than that in WKY (at +60-mV: 7.61 ± 0.89-pA.pF vs. 2.61 ± 0.66-pA.pF ). In SHR myocytes, 5-HT caused a greater inhibition (clorgyline-, polyethylene glycol-catalase- and reduced glutathione-sensitive) of BK amplitude than in those from WKY. CONCLUSIONS AND IMPLICATIONS 5-HT caused an increased generation of mitochondrial H O via MAO-A-mediated 5-HT metabolism, which caused a greater inhibition of BK gating in basilar artery myocytes, leading to exaggerated basilar artery tension development in SHR. © 2010 The Authors. British Journal of Pharmacology © 2010 The British Pharmacological Society.