Myotonia-like activity was recorded from various muscles of rats treated with 25-azacholesterol for a period of 2-8 weeks. 50% of myotonic rats showed a decline of amplitude of the evoked muscle potentials of the flexor digitorum muscle on stimulation of the median nerve with supramaximal stimuli at frequencies of 3, 5, 8, 15 or 50 cps. Administration of prostigimine increased neuromuscular failure. This temporary paresis of myotonic muscle resembles the transient impediment of motility of myotonic patients and it is entirely different from myasthenic transmission defect.