In response to oxidative stress, gestational diabetes mellitus (GDM) placenta releases less 8-isoprostane and tumour necrosis factor (TNF) alpha. The effect of oxidative stress on other cytokines and antioxidant gene expressions are unknown. The aim of this study is to further explore the antioxidant status and effect of oxidative stress in GDM tissue. Human placenta, omental and subcutaneous adipose tissue from women with and without GDM were exposed to hypoxanthine (HX)/xanthine oxidase (XO). Cytokine release was analysed by ELISA and cytokine and antioxidant gene expression by RT-PCR. Catalase (CAT) and glutathione reductase (GSR) mRNA expression was higher in GDM (n=18) compared with normal (n=23) placenta. There was no difference in glutathione peroxidase and superoxide dismutase mRNA expression. Antioxidant gene expression was unaltered between normal (n=18) and GDM (n=10) adipose tissue. HX/XO treatment significantly stimulated cytokine release (13/16 cytokines) and cytokine mRNA expression, and decreased antioxidant gene expression (CAT and GSR) in human placenta from normal pregnant women. In GDM placenta, HX/XO only significantly increased the release of 3/16 cytokines, while there was no effect on antioxidant gene expression. In normal and GDM adipose tissues, HX/XO increased proinflammatory cytokine and 8-isoprostane release, while there was no change in antioxidant gene expression. GDM placenta is characterised by increased antioxidant gene expression, and is less responsive to exogenous oxidative stress than tissues obtained from normal pregnant women. This may represent a protective or adaptive mechanism to prevent damage from further oxidative insult in utero as indicated by increased tissue antioxidant expression.