Abstract 4-Aminopyridine and guanidine were administered intraperitoneally to mice during the complete flaccid paralysis induced by treatment with ruthenium red (RuR). At 1–8 min after 4-aminopyridine or guanidine the animals had recovered completely from the paralysis, whereas the control mice injected only with RuR remained paralytic for at least 60 min. Intraperitoneal injections of LaCl 3 had no apparent effects on animal motility and did not reverse the paralysis produced by RuR. However, when La 3+ was administered 30 min prior to RuR the occurence of flaccid paralysis was totally prevented. The results obtained are discussed in terms of the possible antagonist effects of the compounds used on acetylcholine release at neuromuscular junctions.