The sucrose gap technique was used to study the long positive potential (P-potential) in a curarized cat superior cervical ganglion. The frequency of stimulating the preganglionic trunk optimal for P-potential production was 30–40 impulses/sec at a stimulus series duration of 1 sec. Proserine in low concentrations (1–5 µg/ml) increased amplitude and especially duration of the P-potential. Atropine (0.5–2 µg/ml) blocked it completely. Adrenaline and noradrenaline (10–50 µg/ml inhibited both the negative potential (corresponding to the fast EPSP of neurons) and the P-potential in equal measure. The nature of dependence of P-potential amplitude on value of the membrane potential was also studied. It was found that the P-potential is inhibited in solutions with low potassium ion content, and that amplitude of the P-potential rises with an increase of intracellular sodium concentration. The rate of its increase rises with an increase of temperature. Under the influence of strophathin, the P-potential is inhibited. The data obtained support the hypothesis that the P-potential is determined by synaptic activation of the electrogenic sodium pump.