The author presents the biochemical, physiopathological and anatomo-pathological bases of the treatment of myasthenia gravis by ACTH. In this disease there is a reduction of the synthesis of acetylcholine in the body; ACTH stimulates this synthesis, diretly by activation of cholinacetylasis, or indirectly, through reduction of the mass of lymphoid tissues, especially of the thymus, which are responsible for the elaboration of substances which reduce the synthesis of acetylcholine. The author used ACTH "Armour" and Cortrophine "Organon", in the doses of 2.5 to 25 mgm., diluted in 250 to 1,000 ml. of a 5% glycose solution; administration was always by intravenous way, in an average rate of 20 drops/minute, during 8 hours. As associate medication all patients received Prostigmin, which was later replaced, in some cases, by Mestinon or Mytelaze. As accessory medicaments were used potassium chloryde (2 to 8 grams per day) and ephedrine sulfate (25 mgm. 3 times a dayly). Patients were kept in an hyperproteic and achlorated diet and all precautions indicated when using ACTH were employed. Ten patients with myasthenia were studied. All patients had been treated before with anticholinesterasic drugs (Prostigmin, Mestinon, Mytelaze) in proper doses and their reaction to this therapy decreased gradually. In some cases other treatments had been tried thymectomy, denervation of the carotid sinus, radiation of the thymic region) without any result. Spontaneous remission in this group of patients were exceptional and for only short periods. Evolution was followed up from the clinical point of view. In all cases complete or almost complete remission occurred after a variable dosage of ACTH. In case 2, for instance, only after the 3rd series of ACTH remission of a severe symptomatology to a slight one was attained. In 6 of the 10 cases myasthenic symptoms became worse during the first 10 days of treatment. The author considers intravenous administration of ACTH as an important contribution to the therapy of myasthenia gravis, being especially indicated in the severe forms, which does not decrease through the use of anticholinesterasic drugs usually employed.