This study examines the site of origin and possible etiology of ventricular premature beats (VPB) in patients with mitral valve prolapse. Ten patients with mitral valve prolapse documented by echocardiogram from the study group. All patients had prolapse of the posterioir leaflet and three additionally had anterior prolapse. There were eight females and two males, with a mean age of 29.1 +/- 11.1 years. All patients were having unifocal VPBs at rest. A vectorcardiogram (VCG) was taken of the VPB by a technique which allowed all VCG loops to be written from the same beat. The VCG analysis indicated that the VPB forces were directed anteriorly, inferiorly, and to the left in six patients. In two patients the VPB was directed posteriorly, inferiorly, and to the left, consistent with right ventricular origin. One of these patients had episodes of ventricular tachycardia. One was anterior, superior, and to the left, and one was markedly anterior, superior, and to the right. In all patients the initial portion of the QRS was inscribed slowly. The three patients with additional anterior prolapse did not show a common difference from those with isolated posterior prolapse. It is concluded that: (1) The majority of these VPBs originate from the posteriorbasal portion of the left ventricle. (2) They originate in the myocardium and not in the Purkinje tissue. (3) There is no relationship between the location of prolapse and the VPB morphology.