Objective To evaluate the effects of gender and age on median-effective target plasma concentration(EC50)of propofol administered by target controlled infusion(TCI)causing respiratory depression.Methods Eighty ASA Ⅰ or Ⅱ patients aged 40-79 yr,with body mass index 18-25 kg/m2.undergoing general anesthesia were divided into 4 groups(n=20 each):1 middle-aged male group(MA);Ⅱ middle-aged female group(FA);Ⅲold male group(MO) and Ⅳo ld female group(FO).No premedication was administered.Propofol Was administered by TCI for 15 min,using TCI system incorporating Marsh pharmacokinetic model.EC50 Was determined by up-end-down sequential trial.The target plasma concentration(Cr)was set at 3.1μg/ml in the first Patient in each group.Each time Cr increased/decreased by 10%in the next patient depending on whether or not the respiratory depression occurred.Respiratory depression was defined as RR<8 bpm,Vr≤5 ml/kg,end-tidal PCO2≥50 mm Hg,SaO2≤94%and/or apnea≥15s.Results The EC50 and 95%confidence interval of propofol TCI causing respiratory depression were 6.40(6.09-6.72)μg/ml in group MA,5.93(5.54-6.34)μg/ml group FA,4.58(4.32-4.91)μg/ml in group MO and 4.37(4.14-4.61)μg/ml in group FO.EC50 was significantly lower in group FO than in group FA and in group MO than in group MA,but there Was no significant difference in EC50 between group MA and group FA or between group MO and FO. Conclusion The potency of propofol given by TCI causing respiratory depression is increased in the old patients as compared with the middle-aged patients and is not related to sex.