Thirthy patients who were not suffering from any neuromuscular disease received therapeutic doses of ampicillin, erythromycin and tetracycline hydrochloride; electromyographic recordings were performed before, during and after the administration of these antibiotics. An electromyographic myasthenic-like picture was obtained during ampicillin and erythromycin administration; this picture normalized after the discontinuation of the antibiotics. A marked facilitation in the size of the muscle action potential during supramaximal repetitive nerve stimulation was observed in control patients from the first 2 groups. This finding points to a presynaptic effect of erythromycin and ampicillin. Tetracycline hydrochloride did not alter the electromyographic recording.