The present study has compared the abilities of clozapine, haloperidol, chlorpromazine and loxapine to induce dopamine (DA)-receptor hypersensitivity in rats, as measured by the apomorphine response after withdrawal of the antipsychotic drugs. Haloperidose during 1-2 weeks after withdrawal. Clozapine, given prior to apomorphine, reduced the responses of the haloperidol and loxapine groups to the control level. The effects of haloperidol and clozapine were quantified in rats with unilateral striatal lesions. Biochemical investigations showed that tolerance developed to the increase in striatal homovanillic acid (HVA) after chronic treatment with haloperidol, chlorpromazine and loxapine, whereas clozapine (20 mg/kg p.o.) failed to affect the HVA content, and no tolerance developed to the increase seen at 80 mg/kg. Cross-tolerance to the rise in HVA was seen with haloperidol, chlorpromazine and loxapine, but chronicc pretreatment with clozapine failed to affect the rise in HVA induced by a singel dose of the former compounds.