In rats the effects of chronic treatment with thioridazine (5 mg/kg orally administered for 22 days) were studied by means of behavioural supersensitivity to apomorphine and by means of dopamine (DA) receptors quantitative autoradiography. Locomotion and stereotypies induced by apomorphine increased after thioridazine chronic administration, whereas grooming behaviour decreased. Autoradiographic data showed an increase in DA receptors density both in the striatum and in the olfactory tubercle, to which the increase in stereotypies and locomotion could be respectively attributed. DA receptors increased also in the medial and dorsal frontal cortex. Moreover a decrease in DA receptors density appeared in the nucleus accumbens septi and in the lateral frontal cortex. Receptors decrease found in these regions might be associated with thioridazine-induced chronic inactivation of A10 DA neurons, to which the antipsychotic effect of the drug is attributed.