1. The tunica media of the proximal segments of the pulmonary vein is made up of cardiac muscle cells. The electrical activity of this cardiac portion of the pulmonary vein was studied with intracellular micro-electrodes in isolated preparations. 2. All-or-none action potentials were recorded in all cardiac muscle cells in response to electrical stimulation. These action potentials did not propagate into the adjoining smooth muscle cells. 3. There were differences in properties between cells at the distal end of the cardiac pulmonary vein close to the smooth muscle and those close to the heart. In quiescent preparations, cells at the distal end had resting potentials which averaged 66 mV compared with 71 mV at proximal sites. Action potentials were also smaller and shorter in duration in cells at the distal end. 4. In spontaneously active preparations, pace-making potentials were observed in cells at the distal end while cells close to the heart showed a stable diastolic potential between action potentials. The spontaneous activity of the pulmonary vein was influenced by both inhibitory and excitatory nerves. 5. The interaction between the pulmonary vein and atrial activities was studied in spontaneously active atrial-pulmonary vein preparations. Action potentials recorded from the pulmonary vein always followed those from the right atrium. Electrical stimulation at the pulmonary vein generated action potentials that propagated back into the right atrium. 6. This study demonstrated that isolated pulmonary veins were capable of independent pace-making activity. However the activity of the pulmonary vein was dominated by the SA node in atrial-pulmonary vein preparations under normal conditions.