The psychotomimetic substance, phencyclidine (PCP), was administered chronically to selected members of a primate social colony. PCP induced stereotyped behavior in all treated monkeys at each of three test doses. Furthermore, there was a significant increase in the amount of stereotypy seen during the second and third weeks of treatment compared to the first week. The dopamine receptor blocking agent pimozide antagonized PCP stereotypy. The results are discussed in terms of dopamine mediation of these responses.