Sympathetic nerve-adrenergic receptor systems have been implicated in the pathogenesis of hypertrophic cardiomyopathy (HCM). We studied plasma norepinephrine (NE) levels during exercise and cardiovascular responses to NE in 26 patients with nonobstructive HCM and 26 age- and sex-matched controls. There were no differences in the plasma NE levels at rest (201 +/- 84 vs 233 +/- 100 pg/ml) or in the slope of the log NE-heart rate relationship during exercise between the HCM patient and control groups. When NE was infused intravenously, with increasing doses to 0.20 microgram/kg/min, HCM patients displayed significantly greater increases in mean blood pressure (29 +/- 7 vs 14 +/- 5%, p less than 0.001) and peripheral vascular resistance (39 +/- 7 vs 26 +/- 7%, p less than 0.001) than controls. Although the fractional shortening decreased during NE infusion in controls, it was unaffected in HCM patients, despite a greater elevation of systolic pressure. The responses of left ventricular contractility, estimated by a ratio of systolic blood pressure to end-systolic dimension, were significantly greater in patients with HCM (31 +/- 7 vs 13 +/- 6%, p less than 0.001). These observations indicate that vasoconstrictive responses of the peripheral arteries and inotropic responses of the left ventricular muscle to NE were augmented in patients with HCM, while sympathetic nervous activity remained unchanged. Accordingly, we propose that increased activity of the cardiovascular adrenergic receptor systems, rather than enhanced sympathetic nervous function, may be related to the development of abnormal hypertrophy in HCM.