The effects of 4-aminopyridine (a non-anticholinesterase antagonist of curare-like agents) on the evoked muscle action potentials (EMAP) in a patient with Eaton Lambert Syndrome are reported. On two separate occasions the i.v. administration of 0.31 mg kg-1 and 0.62 mg kg-1 was followed by a 300% increase of EMAP for about 2 h without serious side-effects.