We evaluated the effect of ethynylestradiol (EE) administration (5 mg/kg b.wt. s.c., for 5 consecutive days) on the expression and activity of multidrug resistance-associated protein 3 (Mrp3) in rats. Western blotting analysis revealed decreased Mrp2 (-41%) and increased Mrp3 (+200%) expression by EE. To determine the functional impact of up-regulation of Mrp3 versus Mrp2, we measured the excretion of acetaminophen glucuronide (APAP-glu), a common substrate for both transporters, into bile and perfusate in the recirculating isolated perfused liver (IPL) model. APAP-glu was generated endogenously from acetaminophen (APAP), which was administered as a tracer dose (2 micromol/ml) into the perfusate. Biliary excretion of APAP-glu after 60 min of perfusion was reduced in EE-treated rats (-80%). In contrast, excretion into the perfusate was increased by EE (+45%). Liver content of APAP-glu at the end of the experiment was reduced by 36% in the EE group. The total amount of glucuronide remained the same in both groups. Taken together, these results indicate that up-regulation of Mrp3 led to an exacerbated basolateral versus canalicular excretion of conjugated APAP in IPL. We conclude that induced expression of basolateral Mrp3 by EE may represent a compensatory mechanism to prevent intracellular accumulation of common Mrp substrates, either endogenous or exogenous, due to reduced expression and activity of apical Mrp2.