Time -- a key in experimental and practical medicine.

This paper gives an overview on circadian rhythms which governs visibly all biological features. By illustrative examples of circadian rhythms of man and experimental animals it is shown that: 1. The components of all tissues, organs, and blood undergo circadian changes, manifested in drmatic morphologic and chemical differences. These circadian changes simply do not represent minute "physiologic" variations around a 24-h means. 2. The endogenous nature and the influences of the environmental factors such as food, light, laboratory stress, and seasons acting on circadian rhythms are demonstrated. 3. The different circadian phases determine significantly the in vivo and in vitro results in response to hormones, drugs and poisons. The neccessity to consider circadian rhythms is shown by chronotoxicologic experiments: mortality rates between 0 and 100% may occur after application of identical doses of a given drug at different day times.