OBJECTIVE This study compares the effects of an AT1 angiotensin II receptor antagonist (valsartan) with those of an ACE inhibitor (enalapril) on left ventricular (LV) diastolic function in patients with mild or moderate essential hypertension and no evidence of LV hypertrophy at echocardiography. METHODS A total of 24 patients (16 men, mean age 47 +/- 8 years) underwent radionuclide ambulatory monitoring (Vest) of LV function at rest and during upright bicycle exercise testing before and after two 4-week treatment periods with valsartan (80-160 mg/day orally) and enalapril (20-40 mg/day orally) according to a double-blind, crossover randomization scheme. RESULTS In the overall population no differences between the two treatments were found in LV peak filling rate (PFR) either at rest or at peak exercise. In a subgroup analysis it was found that baseline PFR was normal (= 2.5 EDV/sec) in 12 patients (subgroup A) and impaired (< 2.5 EDV/sec) in the remaining 12 (subgroup B). In both subgroups, valsartan and enalapril induced a significant and comparable reduction of systolic and diastolic blood pressure. In subgroup A, valsartan and enalapril did not induce significant changes in PFR. In subgroup B, valsartan increased PFR both at rest (from 2.0 +/- 0.3 to 2.4 +/- 0.3 EDV/sec, P < 0.01) and at peak exercise (from 4.1 +/- 1.1 to 4.4 +/- 1.0 EDV/s, P < 0.05), whereas enalapril did not change PFR either at rest (2.0 +/- 0.4 EDV/s, P < 0.01 versus valsartan) or at peak exercise (3.7 +/- 1.1 EDV/sec, P < 0.05 versus valsartan). CONCLUSIONS Valsartan-induced renin-angiotensin system blockade is able to improve LV filling in patients with mild or moderate essential hypertension and impaired diastolic function. These findings support the hypothesis of a contribution of the renin-angiotensin system in the control of LV diastolic function in these patients.