The effects of 5-hydroxytryptamine (5-HT) and some related compounds upon electrical events in the cat superior cervical ganglion (SCG) were investigated. method s 78 Urethane and chloralose anesthetized cats were used an the experiments. The left SCG was exposed, its blood :supply left: intact. Ganglionic surface (demarcation) potential between the body of the ganglion and the crushed severed and of the post--ganglionic external carotid nerve was recorded. The evoked monophasic postganglionic mass action potential was recorded from the intact nerve surface. Preganglionic stimulation by - - rectangular monophasic impulses was provided. Drugs were given by i.e. bolus injection, and the following were used: bufotenine, : d;hydroergotamine, DMPP, N,N - dimethyltryptamine (DMT), hexamethonium, 5-HT, 5-hydroxy-alpha-methyltryptaminè (M-5-HT;, LSD, methiothepin, alpha -methyltryptamine, methy ser aide, morphine, ouabain, pilocarpine, psilocybin and tryptamine. Results 5-HT (threshold dose 3-10 nmole or 0.5-2.0 jig 5-HT base) had independent inhibitory and stimulant activities. Inhibition of ganglionic transmission occurred at lower doses than stimulation, and was longer lasting' so with higher doses 1 inhibition--was Drily temporarily masked. Associated with the stimulatory activity was depolarization, small compared to that produced by nicotini-c agents, and this was followed by hyperpolarization. The stimulatory but not inhibitory activity showed tachyphylæis. Desensitization was selective and only after high doses of 5-HT ( 1 mcmole or more) was the stimulant effect of DEEP and KCl reduced. Only the inhibitory effect of 5-HT was possessed by LSD, methysergide, psilocybin and DMT. Bufotenine, DMT and M-5-HT stimulated nicotinic receptors, but M-5-HT did not share the typical 5-HT inhibitory and stimulatory-activity shown by bufotenine. Tryptamine had no 5-HT-like activity, but depolarized and blocked transmission. M-5-HT and alpha-methyltryptamine had an affinity for, but no activity at, excitatory 5-HT receptors, where they blocked the activity of 5-HT. The-stimulant and inhibitory effects of 5-HT were not prevented by methysergide, LSD, and morphine in doses which did not depress postganglionic action potentials. Methiothepin did not antagonize the ganglionic effects of 5-HT.