1. Confocal laser scanning microscopy was used to visualize intercellular transmission of Ca2+ waves in intact rat ventricular trabeculae micro-injected with the calcium indicator fluo-3. 2. Ca2+ waves usually failed to be transmitted from cell to cell. At identified individual end-to-end cell contacts, successful transmission interspersed with failure, which sometimes occurred despite an apparent small spritz of Ca2+ between cells. The probability of cell to cell transmission (P(tran)) was 0.13. 3. Ca2+ waves arose preferentially near junctions of connected cells, where connexin-43 was found, but randomly in enzymatically disconnected heart cells. 4. β-Adrenergic stimulation significantly increased P(tran) (to 0.22) and heptanol, an uncoupler of gap junction channels, significantly decreased it (to 0.045). 5. In regions of high [Ca2+](i) due to damage, wave frequency decreased markedly with each cell-cell junction passed. 6. The Ca2+ permeability of cardiac gap junctions may be regulated, and the low ability of cardiac gap junctions to transmit Ca2+ may help control the spread of Ca2+ from damaged regions.