A multitude of factors influence the prescribing, dosing, and clinical monitoring of long-term drug therapy in elderly patients. These include life-style and environment, possible multiple disease states and concomitant medications, compliance, and continuous changes in physiology, all of which can--with advancing age--gradually alter the pharmacodynamics and pharmacokinetics of drugs. The physiologic changes associated with aging that can affect the absorption, distribution, excretion, and metabolism of drugs are reviewed and related to the clinical use of several drugs that are widely used in elderly patients. Important clinical factors may also magnify or counteract the physiologic changes that occur with respect to altered drug response. Finally, the pharmacokinetic properties of the available oral hypoglycemic agents are described as they relate to selection of an optimal drug in elderly diabetic patients.