ObjectiveTo evaluate the effects of ovarian cycle on median-effective target plasma concentration (EC50) of propofol administered by target controlled infusion (TCI) caused loss consciousness.MethodsForty ASA I or II patients who had age 20 - 39 (28.5 ± 4.8) years and were scheduled for elective gynecologic laparoscopic surgery were divided into 2 groups (n =20 each group) : The follicular phase group (F group, serum progesterone concentration O.311.52 ng/ml) and luteal phase group (L group, serum progesterone concentration 5.16 ～ 18.56 ng/ral).No premedication was administered.Propofol was administered by TCI.TCI system was incorporated with Marsh pharmacokinetic model.EC50 was determined by up-and-down sequential trial.The target phsma concentration (Cp) was set at 3.5 μg/ml in the fh-st patient in each group.Each time, Cp was increased/decreased by 10％ in the next patient depending on whether or not the loss consciousness occurred.ResultsThe ECho and 95％ confidence interval of propofol TCI caused loss consciousness were 4.76 (4.52 -5.00) μg/ml in group F, 4.18(3.88 ～4.52)μg/ml in group L.EC50in group L was significantly lower than in group F (t =6.23, P ＜0.01).ConclusionsAs loss consciousness occurred, median-effective target plasma concentration of propofol in luteal　phase was lower than that in follicular phase.