Dimethylether of d-tubocurarine iodide (DMT) was administered daily, by intraperitoneal route, to adult male rats during 8 weeks. The animals were divided in three groups: the animals of the first group were given 5.5 gama/kg/day of DMT; those of the second group were given 16.6 gama/ kg/day; the rats of the 3rd (the control group), received 1 cm³ of isotonic saline solution. The animals muscular force was evaluated once a week right before and after intraperitoneal injection of the drug. During the course of the trial, it was noted: a)a significant reduction of the animals muscular force, evaluated after disappearance of the DMT curarizing effect; b)measurements of muscular force, performed straight after the injection, showed that a 5.5 gama/kg/daily dose induced an increase of the animals susceptibility to the blocking action of DMT; c) the inhibitory effect induced by DMT 16.6 gama/kg daily dose increased at the beginning but decreased significantly after the 5th week of the trial, despite a progressive decrease in muscular force suggest independence of the two effects. Based on these results, some of the phenomena that may occur at the heginning and during treatment of myasthenia gravis are discussed.