DECAMETHONIUM and other depolarizing drugs have a pharmacological effect at the endplate and adjacent regions of muscle fibres. Various modes of action have been proposed for these compounds; some are based on attachment to surface receptors while others require penetration into the fibres. In the work reported here the location of labelled decamethonium has been sought by means of autoradiograms prepared from frozen sections. With this method the distribution of the drug within the fibre can be determined and the movements of the water-soluble compound during processing can be prevented.