To assess the stability of neural deficits produced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the performance of monkeys on an object retrieval (detour) task was studied. The task required retrieval of a banana slice from a transparent box open on one side and fastened to a tray in front of the cage. The orientation of the open side, position on the tray, and position of the banana in the box were manipulated to vary the difficulty of the trials. Six African green monkeys (Cercopithecus aethiops sabaeus) were treated with MPTP (1.5-1.6 mg/kg cumulative doses over 4-5 days) and compared with 5 saline-treated control monkeys. The MPTP-treated monkeys had no gross neurological deficits but did have motor and cognitive deficits during acquisition of the object retrieval task 8-12 months after treatment (J. R. Taylor, Elsworth, Roth, Sladek,Redmond, in press). Performance on the task was examined for 3 months after it had been learned. The MPTP-treated subjects reached at the barrier (transparent side) significantly more than controls and were less successful at retrieving the reward on the 1st reach than controls. Although they took longer to initiate the reach and had more motor problems than controls, they were as likely as controls to retrieve the reward in the end. These deficits remained stable throughout testing. An opaque but otherwise identical box was used randomly on some trials. MPTP-treated subjects decreased barrier reaches to control levels on trials in which the opaque box was used, whereas motor problems increased compared with trials in which the transparent box was used. The task can detect subtle performance deficits similar to those found in Parkinson's disease.