Differences in EC-coupling between mammalian atrial and ventricular cells and intracellular calcium events associated with mechanical shortening have been reported (1,2). This involves the elementary calcium release event, the calcium spark, that initiates whole cell calcium waves which, in atrial cells, has been shown to be mainly localised to sub-sarcolemmal regions at the cell periphery. It has also been shown however that narrower atrial cells, with a less well organised t-tubular system, display a “U”-shaped calcium wave form which originates at the cell periphery, whilst wider atrial cells, with a more organised t-tubular system display a “W” shaped calcium wave with varying points of origin of the calcium wave (3). As human atrial cells have a different morphology to rat atrial cells, the spatially restricted calcium signal observed in rat atrial cells (1,2) may be of limited clinical significance. To test whether calcium waves in human atrial cells have a spatially restricted initiation site we analysed sparks from atrial cells isolated from human tissue obtained during bypass operations. After enzymatic digestion of atrial appendage, cells were loaded with fluo 4-AM and the spatio-temporal properties of sparks recorded with a confocal microscope in linescan (50-60 Hz) and fast x-y (20-40 Hz) acquisition mode. Origin and IDL software was used for analysis. Two types of cell were observed, healthy resting and calcium-overloaded cells. Calcium sparks were observed at the periphery and the centre in both cell types. Spark summation in both regions produced localised sub-cellular waves with larger summation in overloaded cells. In resting cells sparks were more variable in amplitude and less frequent in central regions. Noradrenaline (NA) produced calcium waves with a linear front. In contrast, spark frequency and amplitude distribution in overloaded cells was more uniform in both central and peripheral regions with “V” shaped spontaneous waves. This suggests that coupling between calcium release units was increased in overloaded cells. These results imply that in resting cells there is a difference in release channel activity at the periphery and centre of the cell giving a different spark frequency. This difference is eliminated in overloaded cells and resting cells stimulated with NA. The morphology of human atrial cells and the uniform shape of waves in stimulated resting cells suggests that they have a more organised t-tubule system than atrial cells from other mammalian species. Observations made in other mammalian atrial myocytes may thus be of limited relevance to human atrial function.