Mathematical models of rabbit atrial and ventricular myocytes that are based on quantitative voltage clamp data from emzymatically isolated cardiac myocytes have been developed. These models are capable of accurately simulating the transmembrane ionic currents recorded in response to a step change in membrane potential (whole-cell voltage clamp response), the nonpropagated membrane action potential (MAP), and the frequency-dependent action potential waveshape changes occurring in response to variations in rate of stimulation. Rectangular pulse, ramp and action potential voltage-clamp measurements of the transmembrane ionic currents have allowed us to model a number of processes thought to be important during repolarization. These computations provide important biophysical insights into the electrophysiological activity of atrial and ventricular cells and their associated intra- and extracellular ionic concentration changes. The present model also has useful predictive capabilities. We have used the model to: (1) estimate the intracellular Ca\sp2+ transient in these myocytes and to compare the relative occupancy of the Ca\sp2+ binding sites in the contractile proteins with known cellular mechanical activity, and (2) predict the response of the atrial cell to potassium current blockade via BaCl\sb2 to the bathing medium.