Assays of drug levels in blood and of other biochemical characteristics of psychiatric patients are being proposed for clinical application, although their utility in practice remains uncertain. Exceptions are the assay of blood levels of anticonvulsants [phenobarbital, phenytoin] and of L. Assays of antidepressant drugs may be especially helpful in the evaluation of unexpected responses or in the avoidance of unwanted toxic effects and promise to permit more efficient predictions of individual requirements. Assays of platelet MAO [monamine oxidase] activity or urinary MHPG [3-methoxy-4-hydroxyphenylglycol] excretion remain clinically less useful. Attempts to correlate blood levels of antipsychotic agents with clinical effects have been disappointing, although newer assay methods may prove more useful.