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4-Nonylphenol (para-nonylphenol, 4-NP), metabolites including linear and branched isoforms of nonylphenol (n-NP and t-NP, respectively), has been considered an endocrine disrupting substance resulting in reproductive dysfunction and increasing reactive oxygen species production in testis, liver, kidney, and brain. However, to date, whether vasculature is susceptible to NP exposure remains to be unclear. In this study, we have investigated the effects of chronic in vivo 4-n-NP exposure on vasoconstriction and vasorelaxation in male rats. After a 20-week 4-n-NP treatment orally at the dosage of 10 and 50 μM in the drinking water, phenylephrine- and potassium chloride-induced concentration-dependent responsiveness assessed by wire myograph were both significantly higher in aorta isolated from 4-n-NP-treated rats compared with control rats, but acetylcholine-induced vasorelaxation was similar between these two groups. In addition, systemic oxidative stress and vascular, but not intestinal, oxidant enzyme activities assessed by lucigenin-amplified chemiluminescence were all markedly higher in 4-n-NP-treated rats. In conclusion, our results suggested that chronic in vivo 4-n-NP exposure augments vascular contractile responsiveness through enhanced vascular oxidant enzyme activity. [ABSTRACT FROM AUTHOR]