We have previously reported that angiotensin II (Ang II) treatment of A10 vascular smooth muscle cells (VSMC) increased inhibitory G proteins (Gi protein) expression and associated adenylyl cyclase signalling which was attributed to the enhanced MAPkinase activity. Since Ang II has been shown to increase oxidative stress, we investigated the role of oxidative stress in Ang II-induced enhanced expression of Gi proteins and examined the effects of antioxidants on Ang II-induced enhanced expression of Gi proteins and associated adenylyl cyclase signalling in A10 VSMC. Ang II treatment of A10 VSMC enhanced the production of O₂ ^-and the expression of Nox4 and P47phox; different subunits of NADPH oxidase, which were attenuated towards control levels by diphenyleneiodonium (DPI). In addition, Ang II augmented the expression of Gi-2 and Gi-3 proteins in a concentration and time-dependent manner, the maximal increase in the expression of Gi was observed at 1-2 hr and at 0.1-1.0 µM. The enhanced expression of Gi-2 and Gi-3 proteins was restored to control levels by antioxidants such as N-acetylcysteine (NAC), tocopherol, diphenyleneiodonium (DPI) and apocyanin. In addition, Ang II also enhanced the Erk1/2 phosphorylation which was restored to control levels by DPI. Furthermore, the inhibition of forskolin (FSK)-stimulated adenylyl cyclase activity by low concentrations of GTPS (receptor-independent Gi functions) and Ang II-, C-ANP_4-23 -(NPR-C agonist) and oxotremorine-mediated inhibitions of adenylyl cyclase (receptor-dependent functions) that were augmented in Ang II-treated VSMC were also restored to control levels by antioxidant treatments. In addition, Gs-mediated diminished stimulation of adenylyl cyclase by stimulatory hormones in Ang II-treated cells was also restored to control levels by DPI. These results suggest that Ang II-induced enhanced levels of Gi proteins and associated functions in VSMC may be attributed to the Ang II-induced enhanced oxidative stress, which exerts its effects through mitogen-activated protein kinase signalling pathway.