Benzodiazepine derivatives are the most commonly prescribed anti-anxiety agents in clinical practice. Six benzodiazepine anxiolytics are now available in the United States. Additional drugs are used in other parts of the world, and many others are in various stages of clinical testing. All these benzodiazepine derivatives have similar neuropharmacological properties--they reduce anxiety, produce sedation and sleep, have anticonvulsant effects, and can produce muscle relaxation. Faced with this bewildering array of drugs from the same class which are very similar in intrinsic effects upon the brain, the clinician may well ask how best to make a rational choice among the available derivatives. Despite neuropharmacological similarities, there are differences among benzodiazepines in patterns of absorption, distribution, and elimination by the human body. These pharmacokinetic differences may in turn lead to apparent differences in clinical action. This review summarizes pertinent pharmacokinetic characteristics of benzodiazepine anti-anxiety agents.