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To investigate how intercellular coupling can be changed during Ca2+ overloading of ventricular muscle, we studied Ca2+ signals in individual cells and the histochemistry of the major gap junction channel, connexin43 (Cx43), using multicellular preparations. Papillary muscles were obtained from guinea pig ventricles and loaded with rhod-2. Sequential Ca2+ images of surface cells were obtained with a confocal microscope. In intact muscles, all cells showed simultaneous Ca2+ transients in response to field stimulation over a field of view of 0.3 × 0.3 mm2. In severely Ca2+-overloaded muscles, obtained by high-frequency stimulation in nonflowing Krebs solution, cells became less responsive to stimulation. Furthermore, nonsimultaneous but serial onsets of Ca2+ transients were often detected, suggesting a propagation delay of action potentials. The time lag of the onset between two aligned cells was sometimes as long as 100 ms. Similar lags were also observed in muscles with gap junction channels inhibited by heptanol. To investigate whether the phosphorylation state of Cx43 is affected in Ca2+-overloaded muscles, the distributions of phosphorylated and nonphosphorylated Cx43 were determined using specific antibodies. Most of the Cx43 was phosphorylated in the nonoverloaded muscles, whereas nonphosphorylated Cx43 was significantly elevated in severely Ca2+-overloaded muscles. Our results suggest that the propagation delay of action potential within a small area, a few square millimeters, can be a cause of abnormal conduction and a microreentry in Ca2+-overloaded heart. Inactivation of Na+ channels and inhibition of gap junctional communication may underlie the cell-to-cell propagation delay.