BACKGROUND: Baricity of the local anesthetics is a major determinant of the distribution of local anesthetics in CSF. The purpose of this study was to assess the value of refractometry in measuring the specific gravity of CSF, local anesthetics and adding solutions, and then comparing these to the density of CSF and local anesthetics. METHODS: CSF samples were obtained from 30 patients during spinal anesthesia. 0.5% hyperbaric tetracaines were diluted with CSF to create a 1:1 to 1:10 density and specific gravity measurements were made at room temperature, using a refractometer. 2% lidocaine was diluted with distilled water, normal saline, 5% and 10% dextrose water to create 2%, 1%, 0.67%, 0.5% and 0% mixtures measured respectively. RESULTS: Specific gravity of CSF was 1.0066 +/- 0.0006 and SG of 0.5% tetracaine was 1.0285 +/- 0.0028. The effect of specific gravity on the concentrations of tetracaine was determined by linear regression with r = 0.9803, y = 1.0060 + 0.0440 X x. Dilutions of 2% lidocaine with adding solutions were distilled water, y = 1.0008 + 0.0078 X x ; normal saline, y = 1.0056 + 0.0053 X x ; 5% dextrose, y = 1.0208 - 0.0022 X x ; and 10% dextrose, y = 1.0436 - 0.0135 X x. CONCLUSIONS: The relationship between concentration and specific gravity of local anesthetics is linear. We conclude that refractometry is a reliable method to predict density of local anesthetics.