Acetylcholine (ACh) caused two types of depolarizations of the soma membrane of bullfrog primary afferent neurons (dorsal root ganglion cells); the one, a rapid transient depolarization, was nicotinic and the other, a long-lasting one, was muscarinic in nature, respectively. The rapid transient depolarization was due to a simultaneous increase in sodium and potassium conductance, whereas the slow one was caused by a decrease in membrane potassium conductance. These results indicate that the soma of bullfrog primary afferent neuron is endowed with nicotinic and muscarinic receptors.