The usefulness of measuring plasma concentrations of drugs in patients is now well established and permits a greater efficacy and safety of treatment since dosage can be adjusted on an individual basis. In monitoring plasma concentrations one must recognize that a number of factors may influence these levels. For instance, levels may be affected by age, body-weight and size, genetical factors, abnormalities of water and electrolyte balance and complications associated with pathological conditions such as renal and hepatic impairment. Likewise, plasma levels will depend on dosage, time and route of drug administration, the bioavailability of the drug and time of blood sampling. In addition, absorption, distribution, metabolism, excretion and concurrent drug administration may profoundly affect plasma drug concentrations.