Collagen extending from chordae tendineae sweeps into each tricuspid leaflet in a loose fanlike arrangement merging with the annulus. Each leaflet contains variable amounts of cardiac muscle in continuity with right atrial muscle; valve fibers are sparse, and scattered myofibers and/or bundles of myofibers usually extend only 2-3 mm into the leaflet body. Valve myofibers are structurally identical to working right atrial muscle but are 2-3 mum in diameter compared to 5-7 mum in working muscle. Electrical stimulation of the low right atrial wall often induces propagated activity in the leaflet. Transmembrane recordings obtained from isolated Tyrode superfused leaflets showed low maximum diastolic potential, action potential amplitude, and rate of depolarization; action potentials usually had prominent plateaus. Excitation of discrete regions in tricuspid leaflets did not excite atrial wall, but valve muscle fibers often developed spontaneous diastolic depolarization and automaticity after stretch or exposure to catecholamines Automaticity was suppressed by acetylcholine. Physiological and pathophysiological significance of tricuspid muscle may reside in its participation in valve closure during ventricular contraction and the fact that it is a site of abnormal impulse formation.