Monkeys exposed to low doses of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) over long periods of time develop cognitive deficits without severe parkinsonian motor signs. In the present study we assessed the effects of the selective and full dopamine D-1 receptor agonist dihydrexidine on delayed response deficits in chronic low dose (CLD) MPTP-treated monkeys. Dihydrexidine caused a dose-dependent improvement in task performance, that could be blocked by the D-1 receptor antagonist SCH-23390. In addition to reducing the number of mistakes made during delayed response performance, dihydrexidine also improved task persistence. These data suggest that dihydrexidine may be useful in treating cognitive as well as motor deficits of parkinsonism.