L-3,4-Dihydroxyphenylalanine (L-DOPA) is the most effective treatment for Parkinson's disease (PD), but its long-term administration is complicated by wearing-off and dyskinesia. UWA-101, a dual, equipotent inhibitor of dopamine (DAT) and serotonin (SERT) transporters, has previously been shown to successfully extend duration of anti-parkinsonian benefit of L-DOPA (ON-time), without exacerbating dyskinesia, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset. However, UWA-101 is racemic and it is unclear whether one or both enantiomers contribute to its actions, and whether a better therapeutic effect might be attained by using a single antipode. In the current study, we synthesised the two enantiomers of UWA-101, R-101 (UWA-121) and S-101 (UWA-122), characterised their pharmacological profiles and administered them to MPTP-lesioned marmosets. Parkinsonism, dyskinesia, psychosis-like behaviours and duration of ON-time were evaluated. UWA-121 is a dual DAT > SERT inhibitor, with an approximate 10:1 DAT:SERT affinity ratio (inhibitory constants (Ki) of 307 and 3830 nM, respectively). In combination with L-DOPA, UWA-121 extended duration of ON-time when compared to L-DOPA/vehicle treatment (by 40%, P < 0.01). UWA-121 also extended duration of ON-time without dyskinesia (by 215%, P < 0.05) and ON-time without psychosis-like behaviours when compared to L-DOPA/vehicle treatment (by 345%, P < 0.01). UWA-121 did not worsen the severity of dyskinesia or psychosis-like behaviours (P > 0.05). UWA-122 is a selective SERT inhibitor (Ki 120 nM, Ki at DAT > 50 μM) and, in combination with L-DOPA, had no effect on ON-time, dyskinesia or psychosis-like behaviours (P > 0.05). These data indicate that dual DAT and SERT inhibitors effectively enhance L-DOPA anti-parkinsonian action without worsening dyskinesia and that compounds with such a pharmacological profile represent promising agents against wearing-off in PD.