Pempidine, and other highly active ganglion blocking agents of the polyalkylpiperidine series, were developed from tertiary alkylamines, themselves weakly active, on the hypothesis that high activity was conferred by the presence in the molecule of a sterically hindered secondary or tertiary nitrogen atom. Pempidine and its n‐ethyl homologue (26539) resembled mecamylamine qualitatively. All three drugs blocked sympathetic and parasympathetic ganglia; this action was slow in onset and protracted. They blocked neuromuscular transmission, but only about one hundredth as powerfully as ganglionic transmission. They caused a fall in amplitude and rate of the isolated heart, and reduced coronary flow. They had local anaesthetic properties in one of four tests used. They caused tremor. All were well absorbed when administered orally. Pempidine was about twice as active as mecamylamine on ganglia, but only about one half to one quarter as toxic as judged by death, growth, induction of tremor, or cardiotoxicity. Compound 26539 was also quantitatively superior to mecamylamine in respect of these safety margins, but unlike pempidine or mecamylamine damaged the pituitary gland and testis when administered daily for several months. The mode of action of the three drugs is discussed: the results give tentative support for the hypothesis that their action is intracellular.