A sensitive and selective high-performance liquid chromatographic method for the determination of FCE 23884 and its 6-nor-derivative (FCE 26506) in plasma has been developed. After buffering the plasma samples, the compounds and the internal standard were extracted with ethyl ether-n-octanol (9:1, v/v), back-extracted into 0.01 M phosphoric acid and then analysed by reversed-phase liquid chromatography. Quantification was achieved by fluorescence detection of the eluate. The linearity, precision and accuracy of the method were evaluated. No interference from the biological matrix was observed. The assay was adequate for the quantification of plasma levels of the two compounds after a single oral dose of 1 mg of FCE 23884 in humans.