1 The mechanism of action of carbachol was studied on freshly dispersed cells of the rat anococcygeus using microelectrodes and patch pipettes. 2 Micro-ionophoretic application of carbachol evoked reproducible depolarizations which were reduced or blocked by atropine (10(-7)-10(-6) M). The time courses of the responses to noradrenaline and carbachol were similar. 3 The reversal potential of the carbachol-induced response was -3.8 mV and similar to the value (-6.2 mV) found for noradrenaline. 4 During the response to carbachol the membrane conductance was increased. At depolarized membrane potentials carbachol evoked biphasic membrane responses suggesting an increase in two separate ionic conductances. 5 With patch pipettes in the whole-cell configuration under voltage-clamp, carbachol produced an inward current at a holding potential of -50 mV. The inward current was associated with an increase in membrane conductance with an equilibrium potential of about 0 mV. 6 It is suggested that muscarinic receptors and adrenoceptors in the rat anococcygeus may activate similar membrane conductances. The most prominent mechanism is an increase in chloride ion conductance.