Sensitive methods were developed for the analysis of dextromethorphan (I) and two metabolites, (+)-17-methyl-morphinan-3-ol (II) and (+)-morphinan-3-ol (III), in plasma as well as dextromethorphan and three metabolites II, III and (+)-3-methoxymorphinan (IV) in urine using high-performance liquid chromatography followed by detection with a fluorometer. Dextromethorphan and its metabolites were extracted from plasma and urine and separated in the reversed-phase mode. The practical lower limits of determination for I, II, and III in plasma were 0.5, 5, and 5 ng/ml, respectively; for I, II, III, and IV in urine, the limits were 20 ng/ml, 0.6 microgram/ml, 0.5 microgram/ml, and 15 ng/ml, respectively. The linearity of the calibration graphs was excellent (r varied from 0.9994 to 0.9999) over concentration ranges of two orders of magnitude.