A model of β-adrenergic and muscarinic cholinergic stimulation of the bullfrog atrial myocyte has been developed that mimics the dose-dependent effects of isoprenaline (ISO) on the action potential duration (APD); i.e., low doses of ISO lengthen the APD, while high doses shorten the ADP. This reduction in APD is modeled as the result of (1) calcium-dependent inactivation of I\sbCa resulting from the enhancement of I\sbCa by ISO and (2) an enhancement of I\sbK due to both an ISO-induced increase in the rate of activation of I\sbK and an increase in peak action potential height. The effect of acetylcholine (ACh) is to reduce the ISO-induced increase in I\sbCa and I\sbK through a reduction in relative (cAMP) as well as to stimulate the ACh-sensitive K\sp+ current I\sbK,ACh. At low (ISO) levels or high (ACh) levels, the muscarinic cholinergic effect dominates over the β-adrenergic effect. However, for a large (ISO) and a small (ACh), this pattern of changes in transmembrane currents is different; in this case the model predicts that ACh can actually increase APD. A distributed parameter model of an idealized bullfrog atrial trabeculum is developed. Individual cardiac cells are resistively coupled end to end via intercalated discs to form an idealized cylindrical cardiac strand encased in a resistive-capacitative trabecular sheath which, in turn, is located in a finite cylindrical volume conductor. A second-order implicit finite numerical integration method is used to calculate the time-varying potentials within the intracellular (V\spi), interstitial (V\spe), and the outer volume conductor (V\spo) media of the concentric cylindrical structure. 'Reduced' cell membrane models which lack the complete complement of transmembrane currents are compared with regard to their accuracy in representing the foot, upstroke, and plateau regions of the propagated action potential in the complete model. A reduced cell membrane model should contain the sodium current I\sbNa, the calcium current I\sbCa and the background rectifying K\sp+ current I\sbK1. A cell membrane model which contains a linear background K\sp+ current I\sbL instead of I\sbK1 produces a poor approximation to the upstroke, plateau and conduction velocity of an action potential. The trabecular sheath reduces the extracellular resistance seen by the cell by shunting current away from highly resistive interstitial medium into the volume conductor medium which is of low resistance, and thereby increases conduction velocity. Finally, the effects of the cholinergic neurotransmitter, acetylcholine (ACh), on both the passive and active properties of the trabeculum are investigated. The addition of ACh to the extracellular medium reduces the space constant and input resistance of the trabeculum, as well as the conduction velocity of electrical activity propagating through it.