We have formulated a gradient model of the rabbit sinoatrial node based on cell-specific ionic models of electrical activity from its central and peripheral regions. The gradient model exhibits a smooth transition in action potential characteristics such as maximum diastolic potential, overshoot potential, upstroke velocity, action potential duration and cycle length, and is suitable for developing higher dimensional models of the right atrium. Spontaneous pacemaker activity is simulated in a one-dimensional monodomain model of the sinoatrial node and adjoining atrial tissue.