Psychotic patients treated with identical doses of antipsychotic drugs have been shown to have great interindividual differences in their steady state plasma concentration. Therefore, monitoring treatment by dosage adjustment alone is of little value. If antipsychotic blood levels can be related to clinical response then their routine measurement may well result in well defined guidelines to individualised optimal dosage.Despite the considerable effort expended in this field and the many interesting testable hypotheses generated, little substantive evidence for an acceptable plasma level monitoring guide has been reported to date. Work on metabolite level profiles, intra- and extracellular drug concentration differences, more detailed clinical rating scales, and improved experimental design, all show great promise for the future. Investigation of the pharmacokinetics and the elucidation of the often complex metabolic pathways of individual antipsychotic drugs are generating the data base required for the rational pharmacotherapy of these most severely ill patients.Until more data are available, routine monitoring of antipsychotic drug plasma levels remains of research interest.