Calcium handling in mammalian atrial myocytes varies considerably from that in mammalian ventricular myocytes (Freestone et al, 2000). Ventricular cells utilise an extensive transverse-tubular (t-tubular) system to transfer the depolarising action potential arriving at the sarcolemma deep within the cell. This triggers a calcium induced calcium release (CICR) process from the intracellular store for calcium, the sarcoplasmic reticulum (SR). The fundamental calcium release events from the SR are called calcium sparks which aggregate together to form whole cell calcium transients which cause cell shortening. Most atrial cells lack a well formed t-tubular system presumably being thin enough for calcium diffusion from the cell periphery to trigger whole cell calcium transients. The presence of a rudimentary t-tubular system in wider atrial cells has been described however (Kirk et al, 2002; Smyrnias et al, 2010). This may mean that more calcium sparks can appear within the interior of these cells than in narrower atrial cells not possessing a t-tubular system. We have used a Zeiss LSM 510 meta confocal microscope for the fast confocal imaging of the entire cell to examine calcium sparks in Wistar-Kyoto rat cardiac myocytes after they have been loaded with Fluo-4AM dye. The location of the calcium spark initiation sites was compared in left and right atrial cells as well as cell width. In quiescent rat ventricular cells, calcium sparks are found throughout the interior of the cell whereas in atrial cells they are initiated mostly from peripheral sub-sarcolemmal regions. We found that the average cell width for left atrial was significantly greater than for right atrial cells (13.8 ± 0.9 µm, n= 10 from 5 animals and 8.6 ± 0.6 µm, n=21 from 10 animals respectively, p<0.0001). Consequently calcium sparks were more frequently observed in the centre of left atrial compared to right atrial cells and more likely to be seen in the periphery of right atrial compared to left atrial cells. 33% of calcium sparks from the right atria occurred in the centre of the cell (n= 24 from 3 animals), whereas 46% of the calcium sparks in the left atria were seen to originate from the centre (n=21, from 4 animals) This implies that the left atrial cells have more calcium sparks deep within the cell because of a more developed t-tubule system compared to the right atrial cells. Calcium spark initiation sites seem to depend on the width of the left and right atrial cells. Left atrial being significantly wider than right atrial cells, have a more heterogeneous distribution of calcium sparks within the cell.