The trial is a temptative experimental model of myasthenia gravis based on a doubtful theory whereby this disease is atributed to circulating curare in blood. Forty rats kept under the same conditions, bom on the same day and weighing around 350 g entered the trial, divided in four groups. Group I was not handled, Group II was injected with saline, 1 ml i.p. Group III was submitted to dimethyl tubocurarine iodide (DMT) 2,8 mcg/kg i.p. and Group IV received 14 mcg/kg, all solutions being administered daily for a six-week period. One week after the last rats were anesthetized and prepared for cyatic-gastrocnemius neuro-muscular recording. Monopolar pulses were given at frequencies of 0.33 p/sec, 70 p/sec, (trains of 10 pulses at 3 sec intervals), 70 p/sec (kept for 15 sec) and again 0.33 p/sec right after tetanus. Curarizing effective dose of DMT was determined by a "third part blind" when a 80% block was attained. When only 10 high frequency stimuli were applied to the nerve, a significant difference (p<0.05) in response was observed: Group I, 46.50 ± 20.00 g+; Group II, 55.25 ± 11.33 g+; Group III, 37.25 ± 10.77 g+; Group IV, 37.00 ± 12.74 g+. Significant differences in muscular force were also observed with sustained tetanus: Group I, 79.00 ± 16.21 g+; Group II, 76.75 ± 15.23 g+; Group III, 59.12 ± 17.38 g+; Group IV, 61.62 ± 14.74 g. Significant higher doses of curare i.v. were necessary in the group injected daily with the highest dose of curare than in any other group (p < 0.01): Group I, 3.62 ± 1.17 mcg/kg; Group II, 3.69 ± 1.21 mcg/kg; Group III, 4.01 ± 0.80 mcg/kg; Group IV, 5.48 ± 1.40. These results show that chronic administration of curare leads to physical weakness and hyposensitivity to the drug, thus suggesting that although the existence of a curarizing drug in the human blood may in fact contribute for the muscular weakness of the myasthenic patient, the blood curare does not play a major role in the pathogenesis of the syndrome since the myasthenic patient is highly sensitive to the injection of any curare.