Plasma concentrations of thalidomide and primary 5-hydroxylated metabolites including 5,6-dihydroxythalidomide and glutathione (GSH) conjugate(s) were investigated in chimeric mice with highly "humanized" liver cells harboring cytochrome P450 3A5*1. Following oral administration of thalidomide (100 mg/kg), plasma concentrations of GSH conjugate(s) of 5-hydroxythalidomide were higher in humanized mice than in controls. Simulation of human plasma concentrations of thalidomide were achieved with a simplified physiologically based pharmacokinetic model in accordance with reported thalidomide concentrations. The results indicate that the pharmacokinetics in humans of GSH conjugate and/or catechol primary 5-hydroxylated thalidomide contributing in vivo activation can be estimated for the first time.