The effect of doxorubicin (Dx) equilibration between plasma and erythrocytes (RBC), prior to entering the liver, on hepatic elimination was evaluated under two conditions: (I) the drug being first equilibrated for 30 min in the perfusate (containing 27% human RBC) before infusion into the liver and (II) the drug being directly infused into the liver. Mean (N = 6) steady-state hepatic extraction ratios (E) under conditions I and II were 0.286 +/- 0.131 (SD) and 0.592 +/- 0.147, respectively. The marked difference in E was attributed mainly to the initial difference in plasma/RBC Dx distribution ratio of the inlet blood, the slow efflux of Dx from RBC into plasma under condition I, and the slow influx of Dx from plasma to RBC under condition II. The results indicate that most Dx molecules in RBC are not available for elimination. Drug equilibration between plasma and RBC may therefore represent an important factor in hepatic first-pass metabolism.