The number of binding sites for tritiated dopamine as well as the affinity of 3H-dopamine for dopamine receptors was studied in rats chronically pretreated with the neuroleptic haloperidol. The rats were given haloperidol for 14 or 21 days and were killed on day 21. It was found that 14 days of haloperidol pretreatment followed by drug withdrawal resulted in a 67% increase in the number of 3H-dopamine binding sites in the striatum and ninefold increase in the affinity constant in the striatum. The same pretreatment regimen had no effect on either the number of receptor sites or the affinity constant in the nucleus accumbens. These results are consistent with the hypothesis that chronic neuroleptic pretreatment results in receptor site hypersensitivity in the striatum, which may be a major factor in the production of tardive dyskinesia.