Toremifene is a triphenylethylene antioestrogen with significant antitumor activity. It is structurally very similar to tamoxifen. Both drugs undergo extensive hepatic metabolism, and tamoxifen is known to inhibit hepatic mixed-function oxidases (MFO). Using the isolated perfused rat-liver model, we investigated the effect of toremifene on the elimination of antipyrine, a standard marker of MFO activity. Perfusate consisted of 20% red cells in a modified Krebs-Henseleit buffer, and 80 ml was recirculated at 14 ml/min for 3 h. High but clinically relevant steady-state toremifene levels of 3 and 10 micrograms/ml were achieved using bolus plus constant infusion into the reservoir. Elimination of 2.5 mg antipyrine was not inhibited by steady-state toremifene, but methanol (maximal perfusate concentration, 1.29%), the vehicle used for toremifene administration, caused a statistically significant increase in the antipyrine elimination half-life (mean, 1.4 +/- 0.2 h for controls vs 2.2 +/- 0.3 h for methanol; P < 0.05, n = 4). Whereas the methanol had no apparent effect on liver viability as assessed by bile flow and perfusate back-pressure, toremifene at a steady-state concentration of 10 micrograms/ml caused a statistically significant decrease in bile flow (value at 180 min, 0.22 +/- 0.05 ml/h as compared with 0.52 +/- 0.06 ml/h in the methanol control; P < 0.05) and a statistically significant increase in perfusate back-pressure (value at 180 min, 17.5 +/- 1.8 cm vs 11.0 +/- 2.6 cm in the methanol control; P < 0.05). Therefore, toremifene used at high doses can impair liver function in the isolated perfused rat liver, but it does not have any effect on antipyrine elimination.