Phytoestrogens have received an increasing interest due to their potential protective effect against a number of human diseases. A method based on liquid-chromatography and negative electrospray tandem mass spectrometry was developed for assaying phytoestrogens in blood plasma. The detection limit was 0.1 to 3.1 nM for various isoflavones, lignans and mycotoxines. In a dairy cow fed whiteclover grass the arterial plasma concentrations of the conjugated forms of enterodiol, enterolactone, daidzein, and equol were 2, 440, 93 and 115 nM, respectively. The concentrations of the non-conjugated forms were 0.4, 16, 0.8 and 1.1 nM, respectively. Formononetin and genistein were detected only in the conjugated forms (1.6 and 22 nM). There were positive net portal fluxes of all phytoestrogens detected. The liver extracted equol, but released daidzein in the non-conjugated form. These preliminary data indicate important differences in liver metabolism of various phytoestrogens in dairy cows.