Dynamics of electrically coupled model sino-atrial (SA) node cells were studied as a function of coupling conductance. Simulation results demonstrate at least four distinct regimes of behavior as coupling conductance is varied: (a) independent oscillation; (b) quasi-periodic oscillation; (c) frequency, but not waveform entrainment; and (d) frequency and waveform entrainment. The conductance of single cardiac myocyte gap junction channels is about 50 pS. These simulations therefore show that as few as four to five gap junction channels between each cell are required for frequency entrainment. Analyses of large-scale SA node network models implemented on the Connection Machine CM-2 supercomputer indicate that frequency entrainment of large networks is also supported by a small number of gap junction channels between neighboring cells.