We have developed a mathematical model of the rabbit atrial myocyte in order to investigate the ionic bases of rate dependent changes in action potential (AP) wave-shape. Postulated mediators of such changes include rate dependencies of ionic currents (incomplete reactivation) and ionic concentration gradients. The model utilizes biophysical data to quantify the specific morphology and electrophysiology of the rabbit atrial cell. Ionic current descriptions incorporate whole-cell voltage-clamp data from enzymatically isolated rabbit cardiomyocytes, and account for the reactivation timecourse of the largest ionic currents. Our model can simulate both the whole-cell voltage-clamp data upon which it is based and the steady-state AP wave-shapes observed over a range of stimulus rates. It also predicts the intracellular [Ca\sp2+]\sbi-transient that accompanies the AP, the "premature stimulus" response, and the effect of rapid stimulation on the AP. These responses provide insight into the electrophysiological mediators of atrial refractoriness and arrhythmia.