Using a liquid chromatographic assay, we measured serum neuroleptic concentrations in eight middle-aged or elderly female inpatients with tardive dyskinesia (TD) and eight controls. All 16 patients were receiving either thioridazine or mesoridazine at stable doses. TD patients were found to have a significantly higher ratio of serum concentration to daily dose of neuroleptics compared with controls. A 1-year follow-up revealed that this ratio did not change appreciably in those patients who continued to receive neuroleptics. Differences in serum neuroleptic levels were not related to peripheral inflammatory activity as indicated by serum alpha-1-acid glycoprotein concentrations. Of the various thioridazine metabolites, sulforidazine (which is reportedly the most potent in terms of affinity for dopaminergic and alpha-noradrenergic receptors) seemed to be significantly elevated in the serum of TD patients as compared with non-TD patients. Our data suggest a need for further pharmacokinetic investigations to study neuroleptic metabolism in patients with TD.