The plasma concentrations of anticonvulsant drugs, and of certain of their biologically active metabolites, tend to be proportionate to the antiepileptic effects of these drugs. Consequently, anticonvulsant drug levels in plasma are monitored to help guide the clinician in managing his patients' epilepsies. In making use of the measurements, the clinician needs to know the relation between plasma level and biological effect for the various drugs. He also needs to have some awareness of simple pharmacokinetic principles. These are important in deciding when plasma levels should be monitored in relation to the patients' clinical state, to the dosage interval, and to change in the dosage of anticonvulsant or other drug. The clinician also requires pharmacokinetic knowledge in altering anticonvulsant drug dosage in his patients, and in interpreting plasma anticonvulsant level data, particularly when the patient is concurrently suffering from non-neurological disease. The ability to monitor plasma anticonvulsant levels has appreciably improved the treatment of epilepsy, but to obtain maximum benefits from the method, both pharmacokinetic insight and clinical wisdom are required.