A method for the determination of ethynyloestradiol in samples of rabbit plasma containing pentobarbital and heparin, the former used as an anaesthetic and the latter as an anticoagulant, has been developed. Quantification was carried out using a reversed-phase high-performance liquid chromatographic (HPLC) method in isocratic mode at room temperature, with electrochemical detection at an applied potential of +1 V vs. Ag/AgCl. Under these conditions, the retention time for ethynyloestradiol was ca. 2.9 min, the average recovery from plasma was 74.5%, and the limit of detection was 10 pg, corresponding to a plasma concentration of 50 pg/ml using 1 ml of plasma. Natural oestrogens, oestriol, oestradiol and oestrone showed peaks that did not interfere with ethynyloestradiol, and retention times of ca. 0.8, 2.4 and 3.4 min, respectively.