When racemic primaquine was administered to rats, the majority of the residual primaquine excreted in urine was found to be the (+)-isomer. Using a liver microsome preparation, there was no selectivity in the metabolism of the (+)- and (-)-isomers; however, a liver fraction containing mitochondria and microsomes did show selectivity. In the latter preparation, there was a marked preference for the conversion of (-)-primaquine to (-)-carboxy-primaquine.