Summary The actions of bretylium tosylate on neuromuscular transmission in the rat phrenic nerve diaphragm preparation have been investigated by electro‐ On the guinea‐pig ileum, threshold doses elicit repeated maximal spike contractions which are blocked by atropine. In the presence of atropine, higher concentrations of ranatensin elicit small contraction spikes superimposed on a relatively weak sustained contraction. These latter two actions are not blocked by increasing the concentration of atropine. Other smooth muscle preparations respond as follows: rat uterus, rhythmic contractions; rat duodenum, relaxation; rabbit aortic strip, contraction. Atropine has no effect on the above responses. The rat aortic strip does not respond to ranatensin. Ranatensin is four times as active as bradykinin on the rat uterus. Ranatensin can be readily distinguished from other known peptides such as angiotensin, bradykinin and the eledoisin‐like peptides, by bioassay.