BACKGROUND: The purpose of this study is to investigate the effects of volume and concentration in a constant dose of subarachnoid bupivacaine on the extent and duration of sensory and motor blocks. METHODS: Forty parturients scheduled for cesarean section were randomly assigned to two groups for spinal anesthesia. In group 1, 3.5 ml of hyperbaric 0.25% bupivacaine (8.75 mg) with 0.25 mg morphine and 10 microgram fentanyl were injected into the subarachnoid space; in group 2, 1.75 ml of hyperbaric 0.5% bupivacaine (8.75 mg) with 0.25 mg morphine and 10 microgram fentanyl were injected. Dural punctures were performed at the L3-L4 or L2-L3 interspaces with a 25 gauge needle in the sitting position after which the patients were turned to the supine horizontal position. The level and extent of the sensory and motor block were measured at 3, 5, 10, and 15 minutes. The onset, duration and regression of sensory and motor blocks were studied. RESULTS: There were no significant differences between two groups in the level and extent of sensory and motor blocks. From onset time to maximal sensory block took 12.1 +/- 3.0 min in group 1, and 13.3 +/- 3.7 min in group 2, and from onset time to complete motor block took 10.2 +/- 2.7 min in group 1 and 11.4 +/-2.6 min in group 2. There was no significant difference in the time taken to complete the regression of seosory block, and the complete resolution of the motor block. CONCLUSIONS: A constant 8.75 mg dose of subarachnoid hyperbaric bupivacaine produced, in all groups, a similar, statistically insisnificant, level and extent of sensory and motor block, in spite of different volumes and concentrations.