Using the tissue extraction procedure in conjunction with the HPLC analysis protocol, blood plasma, liver and brain concentrations of OXY and FLUO were measured 1 h after i.p. injection of equal molar amounts (40 mg/kg OXY; 62.5 mg/kg FLUO). Recovery rates of 97 ± 1.8% (liver), 69 ± 3.9% (plasma), and 35 ± 6.9% (brain) for OXY and 75 ± 1.7% (liver), 71 ± 1.5% (plasma), and 29 ± 7.6% (brain) for FLUO were obtained and their concentrations were expressed as μg/ml (blood plasma) or μg/g tissue (liver or brain). As shown in Fig. 1, drug levels were highest in plasma. The lowest concentration of either drug was found in the brain. While the brain concentration of OXY reached approximately 6% of its plasma concentration, the corresponding brain tissue concentration of FLUO was only 2% of that in the plasma, despite far higher absolute plasma concentrations.