An HPLC method for the determination of dextromethorphan, dextrorphan, methoxymorphinan and hydroxymorphinan in plasma using fluorescence detection (excitation and emission wavelength 280 and 315 nm, respectively) was elaborated. The mobile phase was acetonitrile - 0.01 M KH₂PO₄ (3:2) containing triethylamine (350 µl.l^-1), pH 3,6; flow rate 0,7 ml.min^-1; internal standard betaxolol. For isolation of the substances from plasma, liquid-liquid extraction and re-extraction with freezing out the aqueous phase was used. The recoveries of the substances were 91, 101, 76, 90 and 99 %, respectively. The method was used for determination of the CYP2D6 phenotype in 95 healthy, unrelated volunteers from Czech population.