Tenovus Institute for Cancer Research, Heath Park, Cardiff, CF4 4XX (Received 23 February 1978) The antioestrogenic properties of tamoxifen (trans-1-(p-β-dimethylaminoethoxyphenyl)-1,2-diphenylbut-1-ene) have led to its clinical use in metastatic breast cancer (Cole, Jones & Todd, 1971; Ward, 1973). In common with other antioestrogens, however, the compound has inherent oestrogenic activity in some species (Harper & Walpole, 1967) and it is therefore important to define the plasma concentrations of the drug that are associated with its different actions. The present communication reports the development of an assay capable of selectively monitoring concentrations of tamoxifen in samples of plasma by gas chromatography–mass spectrometry (GC–MS). Briefly, tamoxifen was administered i.m. to gonadectomized mature, female rats at three dose levels (0·3, 3 or 9 mg rat−1 day−1 in 100 μ sesame oil for 4 days). Blood samples were withdrawn 24 h after the final injection and plasma was stored at −20 °C until required for