1. Many studies have shown that administration of d-3, 4-dihydroxyphenylalanine (D-dopa) produces contralateral rotation in hemi-parkinsonian animals comparable to L-dopa, with less potency and slower onset. It was postulated that D-dopa was converted to L-dopa to produce these effects. 2. To investigate the postulated chiral inversion of D-dopa to L-dopa and the related mechanism, an enantiomeric separation method for D- and L-dopa using HPLC was first established. Then, rat kidney homogenates containing D-dopa or L-dopa were incubated and subjected to HPLC to detect traces of respective enantiomer generation. The mechanism of the chiral inversion of d-dopa was explored by direct measurement of the production of L-dopa in kidney homogenates. D-dopa incubations containing different concentrations of an inhibitor of D-amino acid oxidase (DAAO) and an inhibitor of dopa transaminase were measured for L-dopa generation using HPLC. The role of DAAO in the chiral inversion of D-dopa to L-dopa was further investigated by using purified DAAO and mutant ddY/DAAO- mouse kidney lacking DAAO activity. 3. In rat kidney homogenate, D-dopa was, indeed, converted to L-dopa, whereas L-dopa was not converted to D-dopa. Sodium benzoate, a selective inhibitor of DAAO, blocked L-dopa generation in a concentration-dependant manner. In contrast with kidney homogenates of wild-type ddY/DAAO+ mice, those of mutant ddY/DAAO- mice lacking DAAO activity did not convert D-dopa to L-dopa unless exogenous DAAO protein was added. Conversely, when carbidopa, an inhibitor of dopa transaminase, was added to the homogenates, significant inhibition of L-dopa production was noted. 4. These results prove the proposal that d-dopa undergoes unidirectional chiral inversion and further suggest that D-dopa is first oxidatively deaminated by DAAO to its alpha-keto acid and then transaminated by dopa transaminase to L-dopa.