Both succinylcholine (SCh) and decamethonium (C10) cause depolarization, or phase I block, and desensitization, or phase II block. Using a frog nerve—muscle preparation invitro, the authors evaluated the relative capabilities of SCh and C10 to cause the two types of block. Indirectly stimulated muscle mounted on a myograph can produce similar records of tension outputvs. time following bath application of both SCh and C10. Curves obtained with SCh at 3.75 µM, 12.5 µM, and 25 µM, and with C10 at 25 µM, 75 µM, and 150 µM concentrations were comparable. Using this 1:6 concentration ratio of SCh to C10 in studies of transmembrane potential in single cells, it was found that SCh caused greater and more sustained depolarization than C10. Even when SCh concentrations were increased so as to be equimolar, with C10, SCh caused greater depolarization and less desensitization. Compared with SCh, C10 has a limited depolarization capability which cannot be overcome by increasing its concentration. Finally, the desensitization caused by C10 is more difficult to reverse than that caused by SCh.