The effect of the increased plasma protein binding of quinidine on its disposition was investigated in turpentine-treated rats, since turpentine treatment is known to increase the plasma concentration of alpha 1-acid glycoprotein which preferentially binds basic drugs. The plasma free fraction of quinidine 16 and 48 h after turpentine treatment was decreased by 30 and 76%, respectively, compared to the control value. The treatment did not cause liver injury nor alter the hepatic blood flow. The disappearance of quinidine in plasma after an intravenous injection (3.0, 7.0, 12.5 mg/kg) was analyzed by a two-compartment open model in both control and turpentine-treated rats. The blood total body clearance (CLb) of quinidine at 48 h after the treatment was decreased by 30 to 65% in a dose-dependent manner, compared to that in control rats. The distribution volume (Vdss) of quinidine (12.5 mg/kg) at 16 and 48 h after turpentine treatment was decreased by 30 and 79%, respectively. Hepatic extraction ratio (HER) of quinidine, which was determined at steady state blood concentrations from 0.5 to 2.3 micrograms/ml, was decreased from 0.8 to 0.35 with an increase in the quinidine concentration in control rats. The HER value 48 h after turpentine treatment was consistently reduced by 15 to 40% in a concentration-dependent manner compared to the corresponding control value. These findings indicate that the increased plasma binding of quinidine caused a reduction of HER of the drug, and the reduced HER resulted in the decrease in CLb in turpentine-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)