Cremophor EL, a surfactant used in the clinical formulation of cyclosporine and paclitaxel, will reverse the multidrug resistance (MDR) phenotype in vitro. As other MDR modulators can alter the pharmacokinetics of cytotoxic drugs, the aim of this study was to examine the effect of Cremophor and another MDR-reversing surfactant, Tween 80, on the hepatic elimination and biliary excretion of etoposide. Using the isolated perfused rat-liver model with 80 ml recirculating perfusate containing 20% red blood cells and 4% bovine serum albumin, etoposide (1.6 mg) with and without Cremophor (800 or 80 mg) or Tween 80 (80 mg) was given into the perfusate reservoir, and perfusate and bile samples were collected for 3 h. Etoposide was measured by high-performance liquid chromatography (HPLC) and Cremophor was measured using a bioassay. Both surfactants changed the etoposide elimination profile from biphasic to monophasic. High-dose Cremophor increased the AUC (from 334 +/- 23 to 1540 +/- 490 microgram min ml(-1), P<0.05) and decreased the total clearance (from 4.8 +/- 0.3 to 1.1 +/- 0.3 ml/min, P<0.05) and biliary clearance (from 2.6 +/- 1.1 to 0.5 +/- 0.2 ml/min, p<0.05) but decreased the elimination half-life (from 62 +/- 17 to 40 +/- 5 min, P<0.05) and volume of distribution (from 424 +/- 85 to 65 +/- 19 ml, P<0.05). Low-dose Cremophor and Tween 80 caused intermediate effects on these parameters that were statistically significant for total clearance, half-life, and volume of distribution. Cremophor had no adverse effect on liver function, whereas Tween 80 caused haemolysis and cholestasis. The initial high-dose Cremophor perfusate concentration was 0.8 mg/ml, which previous studies have shown to be clinically relevant and close to the optimal level for MDR reversal in vitro (1.0 mg/ml). Cremophor may be a clinically useful MDR modulator, but it may alter the pharmacokinetics of the cytotoxic drug.