1. Evoked ganglionic action potentials and drug-induced changes in resting potential were recorded by the sucrose-gap method. The height of the action potential was 10-20 mV.2. Since the amplitude of the P wave is inversely proportional to the external potassium concentration between 0.5 and 12 mM, the P wave is probably caused by a phase of increased permeability to K(+).3. In the absence of an anticholinesterase, the depolarizing actions of choline and acetylcholine on the ganglion are almost equal, whereas acetylcholine is 2-3 times more potent than choline on the fibres of the internal carotid nerve. In the presence of eserine, acetylcholine induces a depolarization of the ganglion of up to 14 mV, while increase in [K(+)](o) causes a depolarization of up to 55 mV.4. Although the post-ganglionic fibres in the distal pole of the ganglion are depolarized by drugs, for example, acetylcholine, they contribute little to drug-induced potential changes and probably little to evoked action potentials.5. The hyperpolarization which occurs after exposure of the ganglion to acetylcholine is not due to a phase of increased permeability to K(+).