To study Mrp2 function in integrated systems without confounding high genetic background variability, Mrp2-deficient congenic rats were generated. We investigated the contribution of Mrp2 for biliary and renal elimination of the analgesic M6G. Steady state M6G plasma concentrations (i.v. dosage 0.8 mg/kg*h) were 1.09 ± 0.18 mg/ml in controls and 2.11 ± 0.32 mg/ml in Mrp2-deficient rats (p < 0.001). Biliary M6G excretion was 2 orders of magnitude less in Mrp2-deficient than in control rats. M6G liver concentration was 40 % less in Mrp2-deficient than in control rats. Hepatic mRNA expression of the basolaterally localized and M6G-transporting Mrp3 was seven- fold higher in Mrp2-deficient animals accompanied by a less pronounced Mrp3 protein expression difference suggesting increased basolateral M6G secretion by the Mrp2-deficient liver. Urinary M6G excretion was increased by factor 2 in Mrp2- deficient rats. Renal M6G clearance was 1.44 ml/min * g kidney weight in controls and 1.78 ± 0.13 ml/min * g kidney weight in mutated rats (p < 0.05) with fractional M6G excretion close to unity in either group. We conclude that Mrp2 is critical for biliary elimination of M6G. At therapeutic plasma concentrations renal elimination of M6G occurs via net filtration and is therefore not compromised by renal Mrp2-deficiency.