We have carried out a prospective study on the use of the muscle relaxant, atracurium besilate, in 10 patients with myasthenia gravis, seven had type IIB and three had type IIA, who were scheduled for thymectomy, using ED95 0.20 mg/kg, and ten non myasthenic patients undergoing cholecystectomy using ED95 0.50 mg/kg. After monitoring neuromuscular transmission, the time for establishing maximum blockade (EMB), clinical efficacy time (CET), recovery index (RI) and total duration time (TDT) were measured and evaluated comparatively. Results showed that in myasthenics patients, EMB was faster, CET an TDT were more prolonged and RI was higher than in non myasthenic patients. We conclude that atracurium at doses of 0.20 mg/kg is currently the non depolarizing muscle relaxant of choice in myasthenic patients because of the excellent muscle relaxation allowing extubation in the operating room without reverting muscle blockade and not needing ventilatory support during postoperative period.