Acetylcholine in concentrations of 10-5, 2 x 10-5, 5 x 10-5 and 10-4 in normal Ringer's solution causes contracture of different muscles of Rana esculenta. The similar effect brought about by KCl solution does not necessarily mean the same basic process leading to similar changes in muscle mechanics. Experiments made on many particular questions of muscle mechanics will possibly clarify the real role of single factors (e.g. transmitters, kations, anions, acids, alkalis. ATP, heat etc.) changing the mechanical state of muscular organs.