BACKGROUND: Mivacurium is a nondepolarizing muscle relaxant and metabolized by pseudo-cholinesterase(pChe). Many reports show fall in pChe activity during pregnancy, so the metabolism of mivacurium may be delayed and muscle relaxation would be prolonged. METHODS: Muscle relaxation of full-term pregnant women(C group, n=10) and nopregnant women(Non-C group, n=10) was maintained by continuous infusion of mivacurium to keep 1st response of TOF at 5+/-1%. After discontinuance of infusion, recovery profiles were measured with accelerography. RESULTS: The Infusion rate of mivacurium to maintain 1st twich response of TOF at 5+/-1% was significantly low in C group comparing with Non-C group(P<0.05). There was no significant difference in pChe activity between two groups. There was no significant difference in recovery index, recovery time(T1 25%-T4 ratio, 0.75). There was a little correlation between the total infusion time and recovery profiles(recovery index: r2=0.37, recovery time: r2=0.28). Strong correlation existed between bolus-TS(time interval from the injection of mivacurium to recovery of 5% twitch hight) and infusion rate(r2=0.76). CONCLUSION: The mivacurium infusion rate of C group to maintain muscle relaxation was significantly lower than Non-C group. There would be many possible reasons including over-estimation of paturient body weight compared with lean body mass, decrease of blood volume due to hemorrhage.