Stercuronium is a conessine derivative patented by Koninklijke Nederlandsche Gist-en Spiritusfabriek N. V. as a competitive neuromuscular blocking agent with antimuscarinic activity. In preclinical models, Stercuronium produced significantly greater inhibition (P < 0.05) of the bradycardia than of the vasodepressor response produced by carbachol. In guinea-pig atria, the negative chronotropic response to carbachol was inhibited to a similar degree to the negative inotropic response by Stercuronium, whereas in bladder and ileum Stercuronium was 17 fold less active as an antimuscarinic drug. The affinity of Stercuronium for the prejunctional muscarinic receptor on sympathetic nerve endings in the rabbit ear artery was similar to that for the muscarinic receptor mediating negative inotropic responses to carbachol in the rabbit left atrium. Unfortunately, in clinical trials Stercuronium producing marked tachycardia in some patients when used as a muscle relaxant.