In the pacemaker of the heart, the sinus node (SN), localised Ca2+ release events (Ca2+ sparks) occur from the sarcoplasmic reticulum (SR) during diastolic depolarization. The resulting elevation of intracellular Ca2+activates the forward mode of the Na+-Ca2+ exchanger (NCX), generating an inward current and accelerating pacemaking. This is referred to as the Ca2+ clock.1 Sick sinus syndrome (including sinus bradycardia) is primarily a disease of ageing. We have previously shown that mRNA and protein expression of RYR2 (SR Ca2+ release channel) is decreased in the sinus node of the aged rat.2 The aim of this study was to determine if the Ca2+ sparks in SN myocytes are altered as a consequence of ageing as this may partially explain the sinus bradycardia associated with age. Single myocytes were isolated by enzymatic digestion of the SN from either 12-14 week or 19-20 month old male C57BL/6J mice (young, n=6 cells from 3 mice; aged, n=4 cells from 3 mice). All procedures were in accordance with the Animals (Scientific Procedures) Act 1986. Changes in intracellular Ca2+ concentration, [Ca2+]i, were detected by loading cells with 2.5 µM Fluo-8 AM. Cells were bathed in Tyrode’s solution (containing in mM: 140 NaCl, 5.4 KCl, 1 MgCl2, 5 HEPES, 5.5 glucose, 1.8 CaCl2). Confocal Ca2+ imaging was performed using an Andor Revolution XD confocal system with a Yokogawa CSU spinning disk. Ca2+ sparks were analysed offline from x-y image stacks using xySpark software.3 Data is expressed as mean ± S.E.M. Statistical analysis was performed using a Mann-Whitney test and statistical significance was accepted when P<0.05. In aged SN myocytes, mean Ca2+ spark frequency (measured as sparks/1000 µm2/s) was reduced to 35% of the frequency in young SN myocytes (29.0±7.7 in aged vs 94.3±21.6 in young SN myocytes, P<0.05). Mean spark amplitude (ΔF/F0) was 0.06±0.02 in aged and 0.67±0.36 in young SN myocytes (P<0.05), a reduction of ~90%. Average spark spatial width (FWHM) was ~40% greater in aged SN myocytes (4.15±0.2 in aged vs 2.9±0.1 in young SN myocytes; P<0.01). Spark mass was lower in aged SN myocytes (4.11±2.2 in aged vs 14.3±4.1 in young myocytes; P<0.05). Ca2+ spark duration was unchanged between aged and young SN myocytes. These preliminary results show that ageing alters Ca2+ spark frequency and amplitude in the mouse SN. The altered Ca2+ spark characteristics in aged mice could be related to a reduction in RYR2.2 The changes may at least partially explain the sinus bradycardia observed during ageing and the development of sick sinus syndrome.