We studied the ultrastructure and electrophysiology of an anterior mitral valve leaflet obtained from the heart of a recipient of a cardiac transplant. The anterior leaflet of this human mitral valve contained atrial muscle in direct continuity with the left atrial myocardium. The muscle fibers were 3-6, in diameter and contained atrial-specific granules. They were arranged in longitudinal bands of two or three muscle cells with intercalated discs at the cellular poles. These groups of cells were separated from one another by abundant loose connective tissue. The muscle fibers had maximum diastolic potentials of -61 ± 8 mV and action potentials with slow upstrokes (Vmax <15 V/sec) and low amplitudes (66 ± 6 mV) occurred during electrical stimulation of the valve. In the absence of electrical stimulation, valve muscle had phase 4 depolarization and initiated automatic impulses at a slow rate. The rate of impulse initiation was increased by epinephrine. During electrical stimulation, epinephrine also caused the appearance of delayed afterdepolarizations. Triggered sustained rhythmic activity could be elicited. Delayed afterdepolarizations and triggered activity were abolished by acetylcholine and verapamil. Our results indicate that cardiac muscle in the human mitral valve can initiate impulses and might be a site of origin of arrhythmias in the human heart.