BACKGROUND: Many factors determine the distribution of local anesthetics in the subarachnoid space. These major factors are dosage of local anesthetics, baricity of local anesthetics, position of patient, contour of vertebral column. The temperature of local anesthetics alters the baricity of local anesthetics. At 20oC, the density of 0.5% plain bupivacaine is 1.0003 and generally act as isobaric solution in the CSF. As its temperature lowers, its baricity increases. METHODS: Forty patients (A.S.A I and II) scheduled for lower extremity operation under spinal anesthesia were randomized into four groups; group I (37oC 0.5% bupivacaine, sitting position), group II (37oC 0.5% bupivacaine, 15o head-down position), group III (4oC 0.5% bupivacaine, sitting position), group IV (4oC 0.5% bupivacaine, 15o head-down position). The patients were placed in the sitting position (Group I, III) or lateral decubitus (Group II, IV) and dural puncture was performed at the L3-4 interspace using a midline approach (25-gauge Quincke spinal needle). A free flow of clear cerebrospinal fluid was obtained before administration of drug (37oC 0.5% bupivacaine in Group I, II and 4oC 0.5% bupivacaine in Group III, IV). Patients remained in the sitting position or 15o head-down position for 3 minutes after injection. Patients in each group received a solution that had been previously equilibrated in a stove to 37oC and in a refrigerator to 4oC for more than 1 day. Syringes used to administer the bupivacaine solution were also equilibrated to 37oC and 4oC, respectively. We checked sensory block level using pin-prick test at every 5 minutes. RESULTS: There was statistic significance in sensory block level between Group I, IV and Group II, III. The maximum sensory block level and the time to maximum cephalad spread of analgesia was the T4 level and 9.6 minutes in Group I, the T5 level and 13.5 minutes in Group IV compared to the T9 level and 21 minutes in Group II, the T10 level and 18 minutes in Group III. CONCLUSIONS: The temperature of 0.5% plain bupivacaine affects sensory block level and time to block. It is concluded that the temperature of the injected solution plays an important role in the sensory spread of 0.5% plain bupivacaine.