In order to elucidate the fraction of sulfoxidation in the over all in vivo metabolism of chlorpromazine (CPZ), the sulfoxidation of CPZ to chlorpromazine sulfoxide (CPZSO) was studied in rats. CPZ (10 mg/kg) and CPZSO (1 mg/kg) were injected into the rat femoral vein, respectively. And the pharmacokinetic parameters were obtained from the plasma concentration-time profiles of CPZ and CPZSO determined by the simultaneous analysis using high-performance liquid chromatography. It was supposed that these drugs were almost metabolized in vivo because the total excreted amounts of CPZ and CPZSO via urinary and biliary route were lower than 1.4% and 10.61% of the administered dose, respectively. And also, it was found that the fraction of systemic clearance of CPZ which formed CPZSO $(F_{mi})$ was 0.115. These results showed that CPZ was sulfoxized by 11.5% in rats and the residue would be metabolized via the other routes.