To examine the electrophysiologic determinants of provoked atrial flutter (AF) in patients with mitral valve prolapse (MVP), studies were performed in 4 groups of patients: group 1 (n = 5), patients with MVP and AF; group 2 (n = 6), patients without MVP but with AF; group 3 (n = 6), patients with MVP but without AF; and group 4 (n = 5), patients without MVP and without AF. P-wave duration, intraatrial conduction, interatrial conduction and effective refractory periods for both the high right atrium and the low right atrium were longer in group 2 than in group 1. The effective refractory period of the low right atrium was longer in group 3 than in group 1. The interatrial conduction interval was longer in group 2 than in group 4. Thus, in patients without MVP, atrial conduction delay is the predominant determinant of AF, whereas differences in right atrial refractoriness appear to be most important to the provocation of AF in the patient with MVP. These differences in atrial refractoriness may be a result of abnormal autonomic influences in patients with MVP.