A high-performance liquid chromatographic (HPLC) method is described for the simultaneous determination of honokiol and magnolol in rat plasma. The plasma was deproteinized with acetonitrile which contained an internal standard (diphenyl) and was separated from the aqueous layer by adding sodium chloride. Honokiol and magnolol are extracted into the acetonitrile layer with high yield, and determined by reversed-phase HPLC and ultraviolet detection. The limits of quantitation for honokiol and magnolol were 13 and 25 ng ml 1 in plasma, respectively, and recovery of both analytes was greater than 93%. The assay was linear from 20 to 200ngml. for honokiol and from 40 to 400ngml-1 for magnolol. Variation over the range of the standard curve was less than 15%. The method was used to determine the concentration-time profiles of honokiol and magnolol in the plasma following rectal administration of Houpo extract at a dose of 245 mg kg-1, equivalent to 13.5, 24.4mg kg-1 of honokiol and magnolol, respectively.