In order to study regional differences in membrane properties within the sinus node, we have investigated the electrical activity of small specimens of tissue (approximately 0.3 mm in diameter) that have been prepared from the central, transitional and peripheral areas of the rabbit sinus node. The action potential upstroke velocity and overshoot as well as the maximum diastolic potential were significantly greater in tissue from the peripheral area. Paradoxically, the rate of spontaneous activity was slowest in tissue from the central area (normally the leading pacemaker site in the intact heart). Recovery of excitability was slowest in tissue from the central area and fastest in tissue from the peripheral area. The depression of electrical activity produced by rapid stimulation (overdrive) was most marked in the central region. Raising the bathing K concentration depresses spontaneous activity and this effect was again most marked in the central region. Tissue from the central area was the least sensitive to acetylcholine in five out of six hearts. Pacemaker shift can occur within the sinus node in response to various interventions, and differences in electrical activity as observed in this study may underlie such changes.