Objective To determine the optimum dose of pentazocine when combined with propofol for gastroscopy in elderly patients.Methods One hundred and forty ASA physical status Ⅰ or Ⅱ patients,aged 6575 yr,scheduled for elective gastroscopy under general anesthesia,were randomly assigned into Ⅰ-Ⅳ groups (n =35 each) using a random number table.Before insertion of the gastroscope,pentazocine 0.2,0.4 and 0.8 mg/kg were injected intravenously in Ⅱ-Ⅳ groups,respectively,while the equal volume of normal saline was given in Ⅰ group.Propofol was then administered by target-controlled infusion (TCI).The half-effective concentration (EC50 of propofol was determined by up-and-down sequential trial.The target plasma concentration (Cp) was set at 3.5 μg/ml in the first patient.Gastroscopy was performed at 5 min after the target effect-site and plasma concentrations were balanced.The response to gastroscopy was defined as positive when body movement and/or bucking occurred during gastroscopy.Each time the Cp increased/decreased by 0.3 μg/ml in the next patient depending on whether or not the response to gastroscopy was positive.EC50 and 95 ％ confidence interval of propofol TCI inhibiting the response to gastroscopy were calculated using Probit analysis.The development of respiratory depression and hypotension was observed.Results EC50 (95 ％ confidence interval) of propofol TCI inhibiting the response to gastroscopy was 2.82 (2.63-3.02) μg/ml,2.78 (2.58-2.97) μg/ml,2.16 (2.00-2.32) μg/ml and 2.03 (1.88-2.19) μg/ml in Ⅰ-Ⅳ groups,respectively.Compared with group Ⅰ,EC50 was significantly decreased in Ⅲ and Ⅳ groups,and the incidence of respiratory depression was increased in Ⅳ group (P ＜ 0.05).Conclusion The optimum dose of pentazocine when combined with propofol is 0.4 mg/kg for gastroscopy in elderly patients.