A sensitive liquid chromatographic (LC) method was developed and validated for the simultaneous determination of dextrorphan and guaifenesin in human plasma using fluorescence detection. Dextrorphan and guaifenesin were extracted from plasma by a liquid-liquid extraction procedure using chloroform containing laudanosine as the internal standard. A cyano column (15 cm × 46 mm i.d., Spherisorb 5-CN) and a mobile phase containing acetonitrile-triethylamine-distilled water (10:1:89, v/v/v) (pH 6) were used. The concentration-response relationship for dextrorphan was found to be linear over a concentration range of 23–515 ng ml⁻¹ with a lower limit of detection of 20 ng ml⁻¹; the accuracy of the method would fall (95% confidence limit) within 9.53% and 11.07% of the true value for the inter-and intra-day, respectively; the inter- and intra-day precision, as measured by RSD, ranged from 1.88% to 30.07% (mean 2.28%) and from 4.69% to 7.51% (mean 5.67%) over the dynamic concentration range of the method (33–326 ng ml⁻¹). The concentration-response relationship for guaifenesin was found to be linear over a concentration range of 181–8136 ng ml⁻¹ with a lower detection limit of 30 ng ml⁻¹; the accuracy of the method would fall (95% confidence limit) within 9.78% and 8.04% of the true value for the inter- and intra-day, respectively; the inter- and intra-day precision, as measured by the RSD, ranged from 2.55 to 6.07% (mean 3.90%) and from 3.12 to 3.90% (mean 3.52%) over the dynamic concentration range of the method (435–6430 ng ml⁻¹). The relative percentage recovery of dextrorphan, guaifenesin, and laudanosine was found to be 96%, 94%, and 88%, respectively. Benchtop and storage stability of the plasma samples were found to be adequate. In addition, the frozen plasma samples were submitted to three filter freeze/thaw cycles without significant change in the stability of guaifenesin and dextrorphan.