&NA;Intracellular recordings were made from eustachian ridge of cat right atrium to determine mechanisms responsible for subsidiary pacemaker automaticity. Pacemaker action potentials exhibited two phases of diastolic depolarization: an initial steeper slope (D1) followed by a more gradual slope (D2). Cesium (1 mM) decreased D1(‐45.6%) to a significantly greater extent than D2(‐33.6%) and increased spontaneous cycle length (SCL) (+37.7%). Tetrodotoxin (10‐6M) had no effect on maximum rate of rise of upstroke, although it increased SCL (+23.9%). Verapamil (0.4‐1.0 &mgr;M) progressively increased SCL by decreasing late diastolic slope, resulting in oscillatory potentials and eventual quiescence. Both norepinephrine (2×10‐9M) and Bay K 8644 (10‐7M) elicited a significantly greater increase in D2than in D1, resulting in a decrease in SCL. Ryanodine (10‐6M) caused a small but significant initial decrease (−3.7%) followed by a progressive increase in SCL (+172%). Ryanodine decreased D2without changing D1, increased maximum rate of rise and overshoot potential, and abolished tension. In the presence of ryanodine, Bay K 8644 progressively increased D1amplitude, resulting in a cyclic pattern of dysrhythmic activity. In the presence of ryanodine, cesium significantly decreased D1(‐39.3%), shifted the late diastolic potential more negative, and increased SCL (+25.7%). These results indicated that multiple mechanisms participate in subsidiary pacemaker automaticity. They include 1) a cesium‐sensitive component that contributes to a greater extent during the initial phase of diastolic depolarization, 2) a component mediated via calcium released from the sarcoplasmic reticulum that contributes primarily during the latter half of diastolic depolarization, and 3) possibly a direct contribution by the slow inward calcium current. (Circulation Research1989;64:648‐657)