A new and simplified high-performance liquid chromatographic (HPLC) method for the determination of the antiestrogenic drug tamoxifen (TAM) and its desmethylated and hydroxylated metabolites in human plasma is described. Specific and sensitive fluorescence detection is achieved by "on-line" photochemical conversion of the TAM structure to a highly fluorescent phenanthrene product using a newly developed post-column photoreactor included in the HPLC system. A highly selective chromatographic separation was established by using unmodified silica with aqueous mobile phase for separation, with the sample preparation step on a small CN-propyl pre-column, included in the HPLC system. Due to the high specificity of the separation and detection system, even small volumes of deproteinized plasma sample can be injected directly without prior sample extraction. The described method permits a very fast and reproducible determination of TAM and its two major metabolites in plasma on a routine basis, down to 100 pg ml-1 concentration.