A reverse phase high performance liquid chromatography assay used for quantitative analysis of tamoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen extracted from human plasma was developed. Plasma samples were spiked with an internal standard, nafoxidine, extracted with 2% butanol in hexane, and activated to fluorescence by exposure to high intensity short-wave ultraviolet (254 nm) light. Aliquots of extracted plasma were then injected onto a C18 reverse phase column and eluted isocratically with a mobile phase of water and triethylamine in methanol. Fluorescence of activated tamoxifen and its metabolites was measured at 266 nm. Peak height ratios were used to quantitate tamoxifen and its metabolites from extracted human plasma specimens.