Radiotelemetric monitoring of hemodynamic parameters was conducted in Wistar rats to study the chronobiological dependence of the effect of clonidine on changes in arterial pressure (AP) and heart rate (HR) induced by immobilization stress. Clonidine injection (20 mg/kg intraperitoneally) caused significant decrease in AD at night both at rest and under stress, whereas at day time its hypotensive effect was manifested only in case of immobilization. The negative cardiochronotropic effect of clonidine was also time-dependent. Thus, chronobiological dependence of the clonidine effects at rest and under stress was disclosed.