The objective of the present study was to determine the effects of early long-term monotherapy with the angiotensin II AT1-receptor antagonist valsartan on the progression of left ventricular (LV) dysfunction and remodeling in dogs with moderate heart failure (HF). Studies were performed in 30 dogs with moderate HF produced by multiple sequential intracoronary microembolizations. Embolizations were discontinued when LV ejection fraction was 30-40%. Two weeks after the last embolization, dogs were randomized to 3 mo of oral therapy with low-dose valsartan (400 mg twice daily, n = 10), to high-dose valsartan (800 mg twice daily, n = 10), or to no treatment at all (control, n = 10). Treatment with valsartan significantly reduced mean aortic pressure and LV end-diastolic pressure compared with control. In untreated dogs, LV ejection fraction decreased (37 +/- 1 vs. 29 +/- 1%, P = 0.001) and end-systolic volume (ESV) and end-diastolic volume (EDV) increased (81 +/- 5 vs. 92 +/- 5 ml, P < 0.001; 51 +/- 3 vs. 65 +/- 3 ml, P = 0.001, respectively) after 3 mo of follow-up compared with those levels before follow-up. In dogs treated for 3 mo with low-dose valsartan, ejection fraction was preserved (37 +/- 1 vs. 38 +/- 2%, pretreatment vs. posttreatment) as was ESV but not EDV. In dogs treated for 3 mo with high-dose valsartan, ejection fraction decreased (35 +/- 1 vs. 31 +/- 2%, P = 0.02) and ESV and EDV increased in a manner comparable to those levels in controls. Valsartan had no significant effects on cardiomyocyte hypertrophy or on the extent of interstitial fibrosis. We conclude that, for dogs with moderate HF, early long-term therapy with the AT1-receptor blocker valsartan decreases preload and afterload but has only limited benefits in attenuating the progression of LV dysfunction and chamber remodeling.