In two cebus (Cebus albifrons) monkeys given weekly oral doses of 0.25 milligram of haloperidol per kilogram, movement disorders appeared 1 to 8 hours after drug administration following the tenth weekly dose. These disorders included oral movements, peculiar postures, writhing, and stretching. Such reactions faded in intensity after the next two doses. Increasing the dose to 0.5 milligram per kilogram has elicited the disorders reliably after each weekly dose for almost 2 years. Similar reactions also developed in a squirrel monkey (Saimiri sciurea) treated weekly with haloperidol and in a third cebus monkey previously maintained for a year on a regimen of 0.25 milligram of haloperidol per kilogram on 5 days per week. These findings suggest an experimental model for determining the etiology of drug-induced movement disorders. They also suggest an unrecognized clinical problem.