1. 1. Chronic administration of neuroleptics results in a .decreased threshold for both amphetamine- and apomorphine-induced stereotyped behavior. This prolonged drug-induced dopamine receptor site hypersensitivity is a workable model of tardive dyskinesia. 2. 2. Guinea pigs treated with haloperidol alone developed a prolonged decrease in the threshold for both amphetamine- and apomorphine-induced stereotyped behavior ( p < 0.05). Those treated with haloperidol and concurrent lithium developed no alteration in threshold. The difference in the threshold in the 2 groups was statistically significant ( p < 0.05) for both amphetamine and apomorphine. 3. 3. Lithium given after haloperidol-induced hypersensitivity of dopamine receptors had developed had no effect on threshold for either amphetamine or apomorphine. 4. 4. These results suggest that prophylactic treatment with lithium might decrease the incidence of tardive dyskinesia, but that lithium would probably be of no value in the treatment of tardive dyskinesia.