To investigate the origin and the conduction of the electrical activity in embryonic and adult mouse hearts we used imaging of Ca2+ -sensitive dyes with a macroscope and an EMCCD camera or with a two photon laser scanning microscope. For acquisition of images at high spatial resolution the temporal resolution proved too low for recording the conduction in beating hearts. We therefore developed a system with increased temporal resolution and improved the signal to noise ratio. The ECG was recorded in parallel to the frame or line trigger. This was used to align the recorded images/lines to a virtual sequence of images of a single heartbeat. Individual heartbeats were averaged to enhance the signal to noise ratio. Since the recordings are asynchronous to the heartbeat, the temporal resolution of the virtual sequence can be increased beyond the acquisition rate by recording of several heartbeats. Spatial resolution was further improved by recording at different lateral or axial positions. To test the system we recorded Ca2+ transients with an EMCCD camera from mouse hearts expressing a novel Ca2+ -sensitive circular-permutated GFP (GCaMP2). The off-line analysis of the ECG triggered images reduced the noise and was able to improve the temporal resolution of atrial and ventricular signals from 10 ms to 2 ms.