In spinal rats physostigmine failed to produce a pressor response even after treatment of the animals with hexamethonium, whereas noradrenaline, McN-A-343 (4-(m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride) and AHR602 (3-acetoxy-1-benzyl-1-methylpyrrolidinium bromide) all produced large pressor effects. In rats anaesthetized with urethane, hexamethonium completely abolished the pressor effect of dimethylphenylpiperazinium but only partially blocked the pressor response to physostigmine; pressor effects of noradrenaline, McN-A-343 and AHR602 were potentiated. Combined treatment with hexamethonium and atropine and with hexamethonium and cocaine, however, completely abolished the pressor effect of physostigmine; simultaneously the pressor effects of McN-A-343 and AHR602 as well as of dimethylphenylpiperazinium were also blocked. P-286 (N-diethylaminoethyl-N-isopentyl-N'N'-di-isopropylurea) produced an early and a late block of the pressor effect of physostigmine; the initial block was due to an adrenergic blocking action while the late block was probably due to a dual action of the drug in abolishing the effects of both the nicotinic and non-nicotinic ganglion stimulants. Pressor responses to physostigmine, McN-A-343, AHR602 and dimethylphenylpiperazinium were abolished immediately after ganglion-blocking doses of nicotine. It is suggested that an unusual ganglionic pathway participates in the mediation of the pressor response to physostigmine in the rat, especially when the established ganglionic pathways are blocked.