Dysfunctions of the autonomic nervous system (ANS) are frequent complications of chronic renal failure. It is not clear, however, whether these dysfunctions occur in patients with acute renal failure. In this study, 15 patients with acute renal failure and 15 age-matched normal subjects were evaluated. Baseline mean blood pressure (MBP) and heart rate were higher (p < 0.01) in patients with acute renal failure than in normal subjects. The Valsalva maneuver and orthostatic stress were used to measure the integrity of the entire baroreceptor reflex arc. Valsalva ratio was lower (p < 0.01) in patients with acute renal failure (1.53 ± 0.09) than in normals (2.3 ± 0.08). During orthostasis, MBP decreased more in patients with acute renal failure (-12 ± 2.7 mm Hg) than in normals (-3.0 ± 2.6 mm Hg). During hand-grip exercise, a measure of integrity of the efferent pathway of the ANS, the increments in MBP (6 ± 2.2 mm Hg) and heart rate (7 ± 1.1 beats/min) in patients with acute renal failure were lower (p < 0.01) than in normals (28 ± 1.9 mm Hg and 21 ± 1.6 beats/min). Baseline plasma levels of norepinephrine (NE) were elevated in patients with acute renal failure; however, the increments of plasma NE during stimuli were reduced. NE infusion at progressive doses of 20, 50 and 100 ng/kg/min produced smaller (p < 0.01) increments in MBP in patients with acute renal failure (1.3 ± 1.0,5.0 ± 0.9 and 10 ± 1.6 mm Hg, respectively), than in normals (2.1 ± 1.9, 11 ± 2.7 and 23 ± 4.7 mm Hg). These abnormalities disappeared with improvement of renal function. The data show that patients with acute renal failure have reduced Valsalva ratio, orthostatic hypotension without an appropriate rise in plasma NE and heart rate, impaired response to hand-grip exercise, and reduced pressor response to NE. These results indicate that: (1) autonomic neuropathy may occur rapidly in patients with acute renal failure; (2) this dysfunction is at least partially due to reduced end-organ response to NE, and (3) the abnormalities disappear with improvement of renal function.