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Previous reports suggest that inflammatory bowel diseases may be accompanied by abnormalities in the neural autonomic profile. We tested the hypotheses that 1) an exaggerated sympathetic activity characterizes active ulcerative colitis (UC) and 2) a reduction of sympathetic activity by clonidine would be associated with clinical changes of UC. In 23 patients with UC and 20 controls, muscle sympathetic nerve activity (MSNA), ECG, blood pressure, and respiration were continuously recorded, and plasma catecholamine was evaluated both at rest and during a 75[degrees] head-up tilt. Autonomic profile was assessed by MSNA, norepinephrine, epinephrine, spectral markers of low-frequency (LF) cardiac sympathetic ([LF.sub.RR]; normalized units) and high-frequency (HF) parasympathetic (HF~R; normalized units) modulation and sympathetic vasomotor control (LF systolic arterial pressure; [LF.sub.SAP]), obtained by spectrum analysis of the R-R interval and systolic pressure variability. Among UC patients, 16 agreed to be randomly assigned to 8-wk transdermal clonidine (15 mg/wk, 9 subjects), or placebo (7 patients). An autonomic profile, Disease Activity Index (DAI), and endoscopic pattern were compared before and after clonidine/placebo. At rest, MSNA, heart rate (HR), [LF.sub.RR], LF/HF, and [LF.sub.SAP were higher and HFRR was lower in patients than in controls. Tilt decreased HFRR and increased MSNA and LF~R less in patients than in controls. Clonidine decreased HR, MSNA, epinephrine, [LF.sub.RR], and increased [HF.sub.RR], whereas placebo had no effects. Changes of the autonomic profile after clonidine were associated with reduction of DAI score. An overall increase of sympathetic activity characterized active UC. Normalization of the autonomic profile by clonidine was accompanied by an improvement of the disease. sympathetic nervous system: microneurography; power spectrum analysis; tilt test.