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In this study the blood pressure, heart rate, plasma noradrenaline and plasma adrenaline responses to various forms of sympatho—neural stress were evaluated in patients with mitral valve prolapse (MVP). Sympathetic reactivity in different subgroups of MVP were related to the degree of ventricular arrhythmia. Thirty—eight patients with mitral valve prolapse and 17 healthy controls were studied. All underwent 24-h ECG recording, 2-D echocardiography, head-up tilt to 60°, pressor tests (sustained handgrip, mental arithmetic, cold pressor) and psychological assessment. The blood pressure, noradrenaline and adrenaline response to stress in patients without premature ventricular contraction were similar to those of the controls. In patients with unifocal premature ventricular contraction (PVC) less than 300/h, responses were similar to normal but were associated with higher plasma noradrenaline levels in the basal state and a diminished response to isometric stress. In patients with more than 300/h unifocal premature ventricular contraction, pairs of premature ventricular contraction, or runs of ventricular tachycardia there were lower blood pressure values in the basal state with reduced blood pressure, heart rate and plasma noradrenaline and adrenaline responses to head-up tilt and sustained handgrip, but marked increases in blood pressure, heart rate and plasma noradrenaline levels during the cold pressor test. Our data suggest different degrees of autonomic involvement in mitral valve prolapse which may be related to the various degrees of arrhythmia which seem to contribute to their symptoms.