The purposes of this work were (1) to develop a high performance liquid chromatographic (HPLC) assay for the enantiomers of thalidomide in blood, (2) to study their inversion and degradation in human blood, and (3) to study the phannacokinetics of (+)-(R)and (-)-(S)-thalidomide after oral administration of the separate enantiomers or of the racemate to healthy male volunteers. The enantiomers of thalidomide were determined by direct resolution on a tribenzoyl cellulose column. Mean rate constants of c h a l inversion of (+)-(R)-thalidomide and (-)-(S)-thalidomide in blood at 37°C were 0.30 and 0.31 h-', respectively. Rate constants of degradation were 6.17 and 0.18 h-'. There was rapid interconversion in vivo in humans, the (+)-(R)-enantiomer predominating at equilibrium. The phannacokinetics of (+)-(R)-and (-)-(S)-thalidomide could be characterized by means of two one-compartment models connected by rate constants for chiral inversion. Mean rate constants for in vivo inversion were 0.17 h-' (R to S) and 0.12 h-' (S to R) and for elimination 0.079 h-' (R) and 0.24 h-' (S), i.e., a considerably faster rate of elimination of the (.-)-(S)enantiomer. Putative differences in therapeutic or adverse effects between (+)-(R)-and (-)-(S)-thalidomide would to a large extent be abolished by rapid interconversion in vivo.