Several investigators have described an interaction between muscle relaxants and hydrocortisones which have showed different results. The exact mechanism of this action is not clear and ther conflicting results have further confusion. The experimental methods were two ways. In the one of method, a group that vecuronium 0.1mg/kg was given intravenously is control and a group that hydrocortisones of various doses(0.3, 0.5 and 1 mg/kg) were administered into vein when T1 was appeared is compared. In the another of method, a control group was anticholinesterase(pyridostigmine 0.12 mg/kg, robinul 0.004mg/kg) were given at the time when T1 reached 25% and a group treated with hydrocortisone 0.5 mg/kg when T1 was appeared is compared. Neuromuscular blockade was measured by recording the twitch response following ulnar nerve stimulation by EMG(ABM, Datex Co. 2Hz 30mA supramaximal voltage). The recovery time from 25% to 75% recovery of twitch height was measured according to recovery index(RI). The results obtained were as follows: `) The RI of control group treated with vecuronium 0.1mg/kg alone was 40.32±20.24 minutes and the group which hydrocortisone 0.5mg/kg was combined, was shorten to 18.79±5.17 minutes, but in the group combined with hydrocortisone 1.0mg/kg and 0.3mg/kg, the RI was also tended to short, but not significant. 2) In the RI of vecuronium 0.1mg/kg, anticholinesterases were given, was 8.46±5.06 minutes and the group combined with hydrocortisone 0.5mg/kg was shorten to 4.77±1.82 minutes significantly. Conclusively, in the small doses of hydrocortisone, there is a effect of antagonism to the vecuronium induced blockade and a potentiated effect to the anticholinesterase activity to the vecuronium.