BACKGROUND: The density of 0.5% bupivacaine solution at 37degreesC is known to be slightly hypobaric. However, its clinical character in spinal anesthesia appears to be nearly isobaric. We tried to find evidence on how the hypobaric 0.5% bupivacaine solution could show the pattern of isobaric distribution in the subarachnoid space. METHODS: The distribution of 0.5% bupivacaine solution containing 0.029 mg methylene blue per milliliter of bupivacaine solution (BMB) was examined in a vertically mounted spinal canal model filled with human cerebrospinal fluid (CSF) at 37degreesC. The CSF was collected from natural drainage bags of 5 brain tumor patients. Three milliliters of the BMB solution at 22degreesC or 37degreesC was injected into the middle of the spinal canal four times with a high speed of injection (0.6 ml/sec) and four times with a slow speed (0.05 ml/sec). Nine 1-ml samples were drawn simultaneously from the 9 sampling sites in the longitudinal direction, each 5 cm apart, then those samples were analyzed for the spectrophotometric absorbance of methylene blue concentration. RESULTS: The concentrations of methylene blue were decreased toward the upper and lower parts of our spinal model with the highest concentrations at the central 5th sampling site, regardless of the temperature and injection speed of the BMB solution. However, the high temperature (37degreesC) and low injection speed respectively contributed to a little more distribution of the BMB solution toward the upper part of the spinal model, and the low temperature (22degreesC) contributed to a little more distribution of the BMB solution toward the lower part of the spinal model. CONCLUSIONS: Our study shows that the distribution of plain 0.5% bupivacaine solution in the subarachnoid space is similar to that of isobaric solutions.