In rats, blood concentrations of sparteine (SP) and relative concentrations of sodium borohydride-reducible metabolite following intra-arterial (i.a.) and portal venous administration of SP-sulphate were estimated up to 200 min. In 24 hr urine, unchanged SP was quantitated. Borohydride-reducible metabolite was measured as the difference in SP concentrations before and after reduction. Administration of SP in a dose of 50 mg/kg SP-sulphate i.a. revealed a blood concentration-time profile which did not allow characterization of the terminal half-life or systemic clearance. Therefore, the dose over the area under the curve up to 120 min after administration, CL0-120app, was defined as an apparent average clearance value over the time interval studied. After a dose of 50 mg/kg the CL0-120app was 34.8 +/- 5.9 ml/min/kg when administered i.a. and 80.4 +/- 7.5 ml/min/kg when administered via the portal vein, thus affording an estimate of 0.64 +/- 0.12 for the hepatic extraction ratio. A possible biliary excretion and enterohepatic circulation was studied in rats with a bile fistula. Although SP levels in blood were lower than in control rats, no SP was excreted in the bile and excretion of SP in urine was even slightly higher, which renders circulation of SP itself unlikely. About 25% of the dose was recovered in 180 min bile as borohydride-reducible metabolite, but the urinary excretion of borohydride-reducible metabolite was not changed. The gradual levelling-off of blood concentration versus time curves may partly be explained by the formation of reactive intermediates in the course of metabolism, which inactivate P-450. In support of this, the intrinsic clearance of orally administered hexobarbital (25 mg/kg) was determined 5 and 50 min after i.a. administered SP-sulphate (50 mg/kg), and decreased from 343 +/- 18 to 220 +/- 36 ml/min/kg (p less than 0.05).