Calcium waves (CaWs) in cardiac myocytes arise from abnormal sarcoplasmic reticulum (SR) calcium release. CaWs are more common in cells isolated from failing than normal hearts. Extrusion of calcium by the sodium calcium exchanger induces after-depolarisations and may trigger action potentials (AP) and potentially initiate arrhythmias. We investigated the timing, cell location and mechanical effects of calcium waves in cardiac myocytes acutely isolated from rat hearts and loaded with fluo4AM. The occurrence of CaW was observed using line scanning confocal microscopy in 10 cells. Trains of normal calcium transients were induced by action potential voltage clamp utilising the permeabilised patch technique (8 animals, 10 cells). Calcium waves were studied during brief pauses (2.5s) between the APs (n=82) and between regular (1Hz) APs (n=12). Contraction during APs was measured from the observed shortening in cell length. The temporal distribution of CaWs during pauses was non-random (Runs test, P < 0.00001), early CaWs being less common than late CaWs (14 of 166 vs. 54 of 166; Fisher’s exact P < 0.0001). When waves occurred in both of two temporally adjacent pauses the two CaWs were more likely to originate in the same cell area than other areas of the cell (chi-squared, P < 0.0001). CaWs occurring between AP associated calcium transients merged with the subsequent calcium transient and were associated with a large reduction in AP induced contraction (mean 29.5%, 95%CI 22.5-36.6%). In conclusion, CaWs exhibit tendencies to arise in particular cell areas, their timing is strongly related to the timing of neighboring calcium transients and the following AP induced contractions are significantly reduced. These findings suggest a novel potential pathological consequence of CaWs and emphasises the importance of the timing of CaW initiation.