A new method has been developed to determine diethylstilboestrol (DES) in bovine urine at the picogramme level. Ion-pair-systems are used for purification and isolation. Following formation of the electrophilic di-heptafluorobutyryl (di-HFB) derivative, identification and quantification were carried out by means of capillary gaschromatography-negative chemical ionization mass spectrometry, using CH4 or CH4-N2O 4 : 1 as reagent gases.