This study examined the relation between autonomic tone and the frequency and duration of supraventricular arrhythmias (SVA) in 44 men (mean age 17 +/- 5 years) with mitral valve prolapse (MVP), confirmed by echocardiography. Ambulatory and short-term electrocardiograms (ECG) were recorded, and transesophageal pacing was performed. Frequency domain heart rate variability (HRV) indices were obtained from short-term ECG recordings at rest and during postural stress. RESULTS: In 14 patients, the index of parasympathetic activity was significantly higher (2.1 +/- 0.01 vs -1.5 +/- 0.01, P < 0.01), and associated with a high incidence of vagally-mediated SVA. The frequency of SV premature beats in patients with parasympathetic prevalence was 58 +/- 86/hour versus 10 +/- 27/hour in patients with sympathetic prevalence (P < 0.05). In 30 patients with high sympathetic activity, the index of sympathetic tone was significantly higher (3.1 +/- 0.01 vs 0.5 +/- 0.01) and the high frequency/low frequency ratio significantly lower (0.294 +/- 0.03 vs 0.581 +/- 0.02, P < 0.05), though the incidence of SVA was significantly lower and duration shorter. The algorithm allowed the noninvasive quantitative assessment of autonomic activity and mechanism behind the development of SVA, which may help in the choice of treatment.