In 4 out of 11 cebus apella monkeys given haloperidol (0.05 - 1.0 mg/kg/d) orally for up to 35 months signs of tardive dyskinesia (TD) hav developed: 1) One monkey developed barely noticeable TD after 4 months, but showed marked and increasing symtpoms of both generalized choreic and buccolingual TD after 8 months. This animal died 3 months after discontinuation of haloperidol. At that time the signs of TD were still prominent. 2) In one monkey bucco-lingual TD appeared after 3 months and was still reversible on discontinuation of haloperidol at 5 months. After a further 12 months of haloperidol, the TD signs proved to be long lasting, possibly irreversible, in this animal. 3) A third monkey showed slight and transient signs of TD at 14 months, but following a further 20 months af haloperidol administration a choreiform syndrome became porminent after drug withdrawal. 4) After 34 months a similar syndrome of choreic movements has been noticed in another animal, increasing after withdrawal of haloperidol. The other 7 monkeys have received haloperidol for 3 - 15 months, without developing any signs of TD. Attacks of acute dystonia have been noticed in all animals, sometimes necessitating anticholinergic medication or decreases in the daily haloperidol dose.