Determination of serum concentrations of certain drugs is becoming increasingly important for optimal patient care. These drugs include many antibiotics, several antiarrhythmics, cardiac glycosides, lithium, phenytoin, some other anticonvulsants, salicylates, and theophylline. Serum level determinations can also be useful for establishing the best individual dosage of benzodiazepines, phenothiazines, and tricyclic antidepressants. On the other hand, information about serum levels is not necessary or useful for many widely used drugs. In general, measurement of serum concentrations is valuable only for drugs whose dosage should be individualized and whose therapeutic and toxic actions are not adequately quantifiable by clinical endpoints. The serum concentration of the drug and of important active metabolites must be accurately measurable, the relation between their concentrations in the serum and the intensity of therapeutic and toxic effects during clinical use must have been clearly defined, and serum levels must always be knowledgeably interpreted in conjunction with careful clinical observation and judgment. Measurements of serum drug concentrations are most often useful during prophylactic drug therapy, in patients with major pharmacokinetic disturbances, and when patients show unusual and unexplained sensitivity or resistance to therapy with a drug.