Hydrochlorothiazide (HCTZ; orally bygavage; 12.5 mg/kg/d; 8 weeks) has improved cardiac function, reduced cardiac interstitial fibrosis and collagen volume fraction, decreased expression of AT1, TGF-β and Smad2 in the cardiac tissues in adult male Sprague Dawley rats. In addition, hydrochlorothiazide reduces plasma angiotensin II and aldosterone levels. Furthermore, hydrochlorothiazide inhibits angiotensin II-induced TGF-β1 and Smad2 protein expression in the neonatal rat ventricular fibroblasts.