['Oxidative stress and fibrosis is implicated in cardiac remodeling and failure. We tested whether allopurinol could decrease myocardial oxidative stress and attenuate cardiac fibrosis and left ventricular diastolic dysfunction in angiotensin II (AngII)-induced hypertensive mice.', 'We used 8-week-old male C57BL/6J mice, in which angiotensin II was subcutaneously infused for 4 weeks to mimic cardiac remodeling and fibrosis. They were treated with either normal saline or allopurinol in daily doses, which did not lower blood pressure.', 'Allopurinol improved diastolic dysfunction in angiotensin II-induced hypertensive mice, which was associated with the amelioration of cardiac fibrosis. However, allopurinol showed no effect on the increased systolic blood pressure by angiotensin II infusion. The ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) [GSH/GSSG] was decreased and malondialdehyde levels were increased in the hearts of AngII-treated mice. Allopurinol also inhibited both the decrease in the GSH/GSSG ratio and the increase in malondialdehyde levels in the heart. Infusion of AngII-induced upregulation of transfer growth factor (TGF)-β1, Smad3 expression and downregulation of Smad7 expression. Treatment with allopurinol reduced cardiac levels of TGF-β1, Smad3, and increased Smad7 expression.'] Does allopurinol attenuate oxidative stress and cardiac fibrosis in angiotensin II-induced cardiac diastolic dysfunction?