Action potentials were recorded from isolated atrial trabeculae of the rabbit using a standard microelectrode technique and analyzed by a phase-plane technique. Muscarinic effects of acetylcholine (ACh) were studied in verapamil-containing solutions (13.2 mumol/l). The shortening of the action potentials by ACh was accentuated in high K+ solutions but depressed in Ca++ excess solutions. ACh-induced conductance was found to be concentration-dependent. From the concentration conductance plots a Hill coefficient of 1 was obtained under all the conditions tested. The maximum conductance calculated from the concentration-conductance plots varied between 11.3 +/- 2.7 microS/cm2 (at 7.5 mmol/l Ca++, 2.7 mmol/l K+ in the external solution) and 46.4 +/- 5.0 microS/cm2 (at 0.1 mmol/l Ca++, 8.1 mmol/l K+). All the effects on the ACh-induced conductance could be described by the reaction scheme for the nicotinic ACh action. From both the experimental findings and the approximation of the measurements using the Katz-Miledi model it is suggested that an increase in the external K+ concentration increased, but an external Ca++ excess decreased, the conductance of the ACh-induced channel.