1 The mechanisms underlying the hyperpolarization which follows depolarization of rabbit superior cervical ganglion cells by acetylcholine, have been investigated and compared with the mechanisms responsible for the hyperpolarizations induced by orthodromic stimulation of the ganglion.2 The amplitude of the drug-induced hyperpolarization (after-hyperpolarization) was diminished when [Na(+)](0) and the duration of the preceding depolarization were reduced.3 In K(+)-free solutions, the amplitude of the after-hyperpolarization was often diminished and its rate of development was reduced. In 12.5 mM K(+)-Krebs solutions, the amplitude and rate of development of the after-hyperpolarization were increased; the potential was still present when the resting potential was at or close to E(K).4 Ouabain (10 muM) prevented or greatly diminished the after-hyperpolarization. The rates of onset and decay of the after-hyperpolarization were reduced in glucose-free solutions.5 It is, therefore, concluded that the after-hypolarization is due to the activity of an electrogenic sodium pump.6 The positive after-potential associated with the ganglionic action potential was increased in K(+)-free solutions and diminished when the resting potential approached E(K), indicating that it is due to a period of increased K(+) conductance. In the presence of high concentrations of hexamethonium (276 muM), the P wave was not selectively depressed by ouabain and has been shown by other workers to be due to a mechanism not involving an increased potassium conductance. It is concluded, therefore, that the positive after-potential, the P wave and the after-hyperpolarization are due to different mechanisms.