Recent work on isolated sinoatrial node cells from rabbit has suggested that sarcoplasmic reticulum Ca²⁺ release plays a dominant role in the pacemaker potential, and ryanodine at a high concentration (30 μmol/L blocks sarcoplasmic reticulum Ca²⁺ release) abolishes pacemaking and at a lower concentration abolishes the chronotropic effect of β-adrenergic stimulation. The aim of the present study was to test this hypothesis in the intact sinoatrial node of the rabbit. Spontaneous activity and the pattern of activation were recorded using a grid of 120 pairs of extracellular electrodes. Ryanodine 30 μmol/L did not abolish spontaneous activity or shift the position of the leading pacemaker site, although it slowed the spontaneous rate by 18.9±2.5% (n=6). After ryanodine treatment, β-adrenergic stimulation still resulted in a substantial chronotropic effect (0.3 μmol/L isoproterenol increased spontaneous rate by 52.6±10.5%, n=5). In isolated sinoatrial node cells from rabbit, 30 μmol/L ryanodine slowed spontaneous rate by 21.5±2.6% (n=13). It is concluded that sarcoplasmic reticulum Ca²⁺ release does not play a dominating role in pacemaking in the sinoatrial node. The full text of this article is available at http://www.circresaha.org.