A new dopamine radioreceptor assay was used to measure serum concentrations of neuroleptics during treatment of 58 patients. Results of this assay are expressed as chlorpromazine equivalents. Serum concentrations varied with different drugs, with extremely high levels in patients receiving thioridazine or mesoridazine. No detectable serum levels were found in 11 patients, probably either due to low drug doses or noncompliance in taking medication. Best results with the assay were obtained in the 22 patients treated with haloperidol. Serum concentrations of haloperidol were correlated with dose and were related to the Brief Psychiatric Rating Scale (BPRS) total pathology score, as well as to BPRS factor scores for thinking disturbance and paranoid disturbance. Serum concentrations of haloperidol were not different among patients with or without the presence of extrapyramidal symptoms or among patients receiving or not receiving concurrent antiparkinson medication.