Twenty-six in-patients treated for schizophrenia, were divided in two groups. The first group received haloperidol, 20-40 mg p.d., and the second, clozapine, 150-250 mg p.d. TRH-TSH test was performed by injecting 0.2 mg TRH. Four drug-free schizophrenic patients gave normal TSH response to TRH, as well as the group of patients treated with haloperidol. Contrary to that, the clozapine-treated group showed a blunted TSH response. The results obtained are discussed in terms of the different pharmacological profiles of haloperidol and clozapine, especially regarding their dopaminergic actions, alpha-adrenergic and serotonergic control upon thyroid axis.