BACKGROUND The median effective dose (ED50) of a drug gives the amount or dose of drug needed to produce effective therapeutic response or desired effect in at least 50% of the population taking it. Our study focused on determining the ED50 required for effective motor block using hyperbaric and plain bupivacaine, and evaluated the influence of baricity on the ED50 required for motor block. MATERIAL AND METHODS A total of 38 patients were randomly assigned into 2 groups according to the baricity of bupivacaine: group P received plain bupivacaine and group H received hyperbaric bupivacaine. The patients were administered 0.5% plain or hyperbaric bupivacaine intrathecally. The dosage of anesthetics in each patient was calculated according to the standard up-down sequential allocation method of Dixon. The first patient in each group received a dose of 7.5 mg bupivacaine, and a dose of 1.0 mg was used as the testing interval. The dose was increased or decreased by 1.0 mg for each patient according to the estimated score of motor block. RESULTS The ED50 required for effective motor block in spinal anesthesia was 7.20 and 10.05 mg in groups H and P, respectively. Their relative motor blocking potency ratio was found to be 0.72. CONCLUSIONS The ED50 for motor block was significantly decreased using hyperbaric bupivacaine intrathecally compared with plain bupivacaine, and the baricity of bupivacaine obviously affected the ED50 for the motor block.