Mammalians circadian pacemaker resides in the paired suprachiasmatic nuclei (SCN). Clock genes are the genes that control the circadian rhythms in physiology and behavior. The effectiveness and toxicity of many drugs vary depending on dosing time associated with 24-hr rhythms of biochemical, physiological and behavioral processes under the control of the circadian clock. However, many drugs are still given without regard to the time of day. Identification of a rhythmic marker for selecting dosing time will lead to advanced chronopharmacotherapy. To monitor the rhythmic marker may be useful to choose the most appropriate time of day for administration of drugs that may increase their therapeutic effects and/or reduce their side effects. On the other hand, several drugs can cause alterations to the 24-hr rhythms, which leads to illness and altered homeostatic regulation. Furthermore, to produce new rhythmicity by manipulating the conditions of living organs by using rhythmic administration of altered feeding schedules or several drugs appears to lead to the new concept of chronopharmacotherapy. One approach to increasing the efficiency of pharmacotherapy is administering drugs at times during which they are best effective and/or tolerated.