A coupled-clock system within sinoatrial node cells (SANC) confers robustness and regulates their normal automaticity: basal cAMP-mediated, protein kinase A-dependent phosphorylation of Ca2+ cycling-proteins enables sarcoplasmic reticulum (SR) to generate spontaneous rhythmic Local, subsarcolemmal Ca2+Releases (LCRs) (“Ca2+ clock”). LCRs activate an inward Na+-Ca2+ exchange current that accelerates the diastolic depolarization promoting the ensemble of surface membrane ion channels (“membrane clock”) to generate the next action potential (AP). Intracellular Ca2+ enters mitochondria through the mitochondrial uniporter and is extruded by the mitochondrial Na+-Ca2+ exchanger. We hypothesized that mitochondrial Ca2+ cycling is coupled to SANC-clocks via its impact on the intracellular Ca2+ cycling. Specific inhibition of Ca influx into and efflux from mitochondria in intact single isolated SANC was effected by Ru360 and CGP-37157, respectively. Changes in mitochondrial Ca2+ content (Cam) were indexed by selective quenching of the fluorescent Ca2+ probe, Indo-1 in the cytosol by Mn2+. Ru360 decreased Cam to 80±8% control and increased the spontaneous SANC AP firing rate to 111±1%. Conversely, CGP-37157 increased Cam to 119±7% control and reduced the spontaneous AP firing rate to 89±2%. Blocking Ca influx into mitochondria increased the average LCR size (measured via confocal linescans images of fluo-4) from 4.2±0.1 to 6.1±0.2µm and reduced the normal LCR period from 317±5 to 274±6ms. In contrast, inhibition of Ca efflux from mitochondria reduced LCR size to 3.6±0.1µm and increased LCR period to 389±7ms. Changes in LCR period by specific inhibition of Ca2+ influx or efflux into and from the mitochondria from 274±6 to 389±7ms predicted (R2=0.84) the concomitant changes in the spontaneous SANC AP cycle length from 349±5 to 462±6ms. We conclude that Ca2+ cycling into and out of mitochondria interact with the SANC coupled-clock system to modulate normal automaticity.