A high-performance liquid chromatographic procedure for the quantitation of [(123)I]Iodomethyl-N,N-diethyltamoxifen (ITX), a radioligand for human breast cancer imaging, in human plasma is described. Separation was effected on a RP-C18 column, using a mixture of acetonitrile-water-triethylamine (70/30/0.5, v/v). ITX was rapidly cleared from human plasma and metabolites appeared as early as 7.5 min p.i. Quantitative assessment of metabolites in plasma over time allowed recalculation of the ITX plasma time-activity curve. Implications of ITX metabolite formation for breast tumour imaging are discussed.