A modification of the Osterrieder-Noma-Trautwein model of the ACh-sensitive K\sp+ I\sbK,ACh current in the rabbit SA node is used to study the behavior of the ACh-sensitive K\sp+ channels in bullfrog atrial and sinus venosus myocytes. The parameters of the modified description of the muscarinic current are chosen to fit available data from the literature on bullfrog atrial myocytes. This model is incorporated into larger mathematical models of bullfrog myocytes that are based on quantitative whole-cell voltage clamp data and simulations are run to discern the effects of this muscarinic channel on the electrical behavior of the cell membrane. Simulations of the electrical behavior of the sinus venosus myocyte predict that the effects of ACh on the pacemaker action potential are dependent on both the phase of ACh delivery and rate of removal of the ACh. Model-generated phase response curves (PRC's), inhibition curves (IC's) and entrainment curves are produced. (Abstract shortened with permission of author.)