The present study was undertaken to determine whether rosiglitazone treatment influences on the agonist-induced or spontaneous regulation of vascular smooth muscle contraction and, if so, to investigate the related mechanism. Stimulants were directly added without any preanesthetic stress or spontaneous vasoconstriction was induced by preanesthetic physical stress where rat aortic ring preparations isolated from rat exposed to preanesthetic stress such as pinch or prick for 30 min were mounted in organ baths and then exposed to contractile agents. Previously and subchronically ingested rosiglitazone decreased Rho-kinase activating agonist-induced contraction but not depolarization- or alpha adrenergic agonist-induced contraction. Moreover, preanesthetic stress induced the stress-induced spontaneous contraction and previously and subchronically ingested rosiglitazone abolished the stress-induced spontaneous contraction. In conclusion, this study provides the evidence and possible related mechanism concerning the vasorelaxing effect of an antidiabetic rosiglitazone as an antihypertensive on the agonist-induced contraction or stress-induced spontaneous vasoconstriction in rat aortic rings regardless of endothelial function.