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The objective of this paper is to get more insight into the circadian regulation of the cardiovascular system, various strains of rats were studied as animal models of human primary and secondary hypertension. Experiments were performed under highly controlled environmental conditions under a light:dark (LD) schedule of 12 h 12 min. To test for the contribution of the biological clock(s), rats and mice were monitored under free-run conditions in total darkness (DD) as well as after a shift of the LD cycle simulating a westward jet-lag. The availability of implantable radiotelemetric devices made it possible to study various cardiovascular functions in freely moving, unrestrained animals. With the use of radiotelemetry, one can simulate the conditions in rodents (changing LD conditions, inducing light shifts) and finally study the effects of drugs, which might be helpful to ameliorate the symptoms.