The study of the electrophyslologlcal activity of the SA node of the heart is currently a subject of considerable Interest In the research community. Information regarding the electrical behavior of the SA node Is by no means complete, yet sufficiently detailed Information Is available In the literature to allow the formulation of a reasonably quantitative model of the primary pacemaker cell membrane, and its vagal Innervation. In this study, the well-known McAlIIster-Noble-Tslen model of cardiac Purklnje fiber Is modified to account for the electrical activity of the SA node. To model the effects of vagal activity on the sinus rhythm a muscarinic channel with dynamics as suggested by Purves and Noma et al. has been added to the basic membrane model. The model mimics the published data quite well and Is capable of characterizing the free running behavior of the SA node, as well as Its response to electrotonic and vagal stimulation.