Chronotherapy addresses the concept that drug delivery can be timed in accordancewith the body's natural biologic rhythms. The development of drug delivery has evolved from the immediate-release systems that required frequent dosing to once-a-day delivery systems that were homeostatic. Now, investigators aere developing new delivery systems that mirror the biologic variations seen in most physiologic and pathologic functions. Using blood pressure as a model, a classic circadian chronbotherapy is one in which drug delivery peaks during the morning hours (to mirror the morning peak in blood pressure) and dissipates during the hours of sleep (because blood pressure normally nadirs during this period) and in which the rate of rise in the drug's plasma level is initially rapid (to mirror the rapid rate of increase in blood pressure that begins before arousal of a patient and is accentuated by their arising). In the future, drug delivery systems utilizing chronobiologic principles may have an important place in clinical practice.