Background There is increasing recognition of myocardial angiotensin-converting enzyme, which is induced with the development of left ventricular hypertrophy (LVH). The potential physiological significance of subsequent increased angiotensin I to II conversion in the presence of LVH is unclear but has been postulated to cause abnormal Ca2+ handling and secondary diastolic dysfunction. Accordingly, we hypothesized that acute angiotensin-converting enzyme inhibition would result in decreased production of angiotensin II and improved active (Ca2+ -dependent) relaxation in patients with hypertensive LVH.Methods and Results Intracoronary (IC) enalaprilat was administered to 25 patients with and without LVH secondary to essential hypertension. Indexes of diastolic and systolic LV function were determined from pressure (micromanometer)-volume (conductance) analysis at steady state and with occlusion of the inferior vena cava. Patients were divided into those receiving high-(5.0 mg, n=15) and low-dose (1.5 mg, n=10) IC enalaprilat during a 30-minute infusion at 1 mL/min. The high-dose patients were further divided along the median normalized LV wall thickness of 0.671 cm/m2. The time constant of isovolumic relaxation (TauL0.671 cm/m2(TauL, 56+-2 versus 44+-2 and 45+-2 milliseconds, respectively, P<.01 by ANOVA) and shortened only in this patient group (Tau sub L, 49+-3 versus 46+-2 and 43+-2 milliseconds, respectively, P<.01 versus baseline and other groups by ANOVA). The improvement in TauLwas directly proportional to the degree of LVH (r=.92, P<.001). Although there was a decrease in LV end-diastolic pressure (23+-2 to 15+-1 mm Hg, P<.01) and volume (86+-8 to 67+-9 mL/m2, P<.05) in those patients with a reduction in TauL, this is due to movement down a similar diastolic pressure-volume relation with no change in chamber elastic stiffness (0.023+-0.002 to 0.025+-0.004 mL sup -1, P=NS).Conclusions Intracoronary enalaprilat resulted in an improvement in active (Ca sup 2+ -dependent) relaxation in those patients with more severe hypertensive LVH. The improvement in active relaxation was directly proportional to the severity of LVH. These results support the hypothesis that the cardiac renin-angiotensin system is an important determinant of active diastolic function in hypertensive LVH. (Circulation. 1994;89:2616-2625.)