Hepatic disposition of trimethoprim (FMP) and sulfamethoxazole (SMX) and the liver distributional volumes were investigated in the in situ per,fused rat liver preparation. Perfusion experiments were conducted using Krebs-bicarbonate buffer delivered via the portal vein (15 ml/min) in a single-pass mode. Erythrocytes (intravascular marker) and Evans blue (extracellular marker) were used for the estimation of liver distributional volumes, and desiccation and freeze-drying methods were used for the estimation of liver water content. TMP and SMX were administered together as a bolus in the presence (1%) and absence of protein. Although SMX profiles displayed a characteristic sharp peak followed by a slower eluting tail in all cases, TMP profiles were dependent on protein; in the absence of protein, the early sharp peak was replaced by a flatter profile with a later peak. Fractional effluent recovery (F; 0.77 vs. 0.82) and hepatic clearance (CLH; 3.44 vs. 2.70 ml/min) for TMP were not influenced by albumin; with SMX, F increased (0.32 vs. 0.60) and CLH decreased (10.2 vs. 6.0 ml/min) with an increase in the perfusate protein concentration. Hepatic extraction of TMP was low (<0.30), whereas it was intermediate (<0.70)for SMX. In addition, distributional volumes and total water content of the liver were successfully determined. Copyright 2008 Prous Science, S.A.U. or its licensors. All rights reserved.