Differences in the availability of d- and l-enantiomers after administration of racemic amphetamine to rats.

1. Rats were treated with racemic amphetamine or separately with the single enantiomers. The two optical isomers were determined in several brain areas, in plasma and urine. 2. The concentration of d-enantiomer significantly exceeds that of the l-enantiomer in brain and plasma but not in urine, following administration of racemic amphetamine. In contrast, when the two isomers are given separately, their brain concentrations are similar. 3. Such a difference does not appear in the brain of mice treated with racemic amphetamine or in the brain of rats pre-treated with SKF 525-A, an inhibitor of amphetamine hydroxylation. 4. The possibility that the l-isomer can interfere with hydroxylation of d-amphetamine is discussed.