Studies attempting to delineate a therapeutic range of blood levels for neuroleptics have yielded conflicting results. The reasons for this are briefly reviewed and a study is described in which a correlation is sought between blood levels of thioridazine and clinical efficacy. The study employs the radioreceptor assay for neuroleptics to detect both parent drug and active metabolites in blood. The results of the study indicate that while dose is a poor predictor of blood levels of medication, blood levels of neuroleptic activity in patients on thioridazine may be very highly correlated with antipsychotic effect.