Summary: The aim of this study was to evaluate a possible relation between the autonomic tone determined by daily urine catecholamine excretion and the incidence of ventricular arrhythmias (VA) in patients with mitral valve prolapse (MVP). The study included 53 patients (31 women and 22 men) aged 19-52 years (mean age 32.7). The diagnosis of MVP was based on medical history, physical examination, and echocardiography. Cardiac arrhythmias were detected by Holter monitoring and classified according to Lown grades. Daily heart rate and duration of corrected QT interval using Basett's formula were also analyzed. Daily urine adrenaline and noradrenaline levels were determined fluorometrically by Von Euler and Lishajko's method. The patients with Lown's grade 111-V VA were evaluated with particular consideration. Student's ttest was used for statistical analysis. On Holter monitoring 26 patients showed VA, including 6 with grade I, 11 with grade II, 2 with grade HI, 4 with grade IV, and 3 with grade V according to Lown's classification. The remaining 27 patients were free of cardiac arrhythmias. Mean daily heart rate ranged from 54-93 beatslmin (73 k 8.44, mean k SD) and corrected QT from 336-494 ms (411 k37.17). Daily adrenaline and noradrenaline excretion for the whole group of patients were 0.01-16.2 pg (2.1 k2.38) and 1.6-31.0 pg (13.1 k7.27), respectively, which was within normal range. However, the patients with serious ventricular arrhythmias showed significantly higher daily adrenaline excretion. Individual analysis of two-thirds of patients with ventricular arrhythmias grade 111-V showed daily urine noradrenaline levels exceeding mean values for the whole group. Our results suggest that in patients with MVP and serious VA, increased autonomic tone may be of great importance.