This study examines the potential for the phytoestrogenic isoflavones, a type of complementary medicine, to be involved in pharmacokinetic interactions in the liver. Rat livers were isolated and perfused to steady state, in single-pass mode, with either 5 μM paracetamol (n = 6), or 5 μM paracetamol with a 50:50 molar mixture of genistein and biochanin A or daidzein and formononetin, at a total isoflavone concentration of 1 and 10 μM (n = 6 for each mixture at each concentration). At 1 μM, neither isoflavone mixture had any effect, while at 10 μM both mixtures decreased the clearance of paracetamol and the formation clearance to paracetamol sulfate. Genistein and biochanin A(10 μM) also increased the biliary extraction of hepatically-generated paracetamol sulfate. Additional livers were perfused with an infusion of 5 μM 14C-paracetamol in the absence (n = 4), or presence, of a 10 μM genistein and biochanin A mixture (n =4). Analysis of washout perfusate and bile samples (up to 30min after stopping the infusion) revealed that the isoflavones reduced the first-order rate constant for paracetamol sulfate transport into perfusate, but not for transport into bile. The results indicate that isoflavones can reduce the formation of paracetamol sulfate and that its enhanced excretion into bile arises from the inhibition of sinusoidal efflux transport.