In four sections, the contribution reviews data on 24-hour variations of cortisol, growth hormone, prolactine, testosterone, LH and FSH in the human plasma. 1) Experiments on isolated subjects show that the 24-h variation of cortisol is based on an endogenous circadian rhythm which can be independent from the rhythm of sleep and wakefulness. 2) The various pattern each of which has been described in good correspondence by several groups of authors, suggest a series of hormones with a strong circadian component (e.g. cortisol) to those with a weaker (testosterone) or even without a circadian component (FSH). 3) The sleep-wake-cycle influences the phase of hormone rhythms by various degrees; therefore, a uniform reference to sleep time seems preferable to a reference to local time in order to compare several rhythms. 4) The rhythms of cortisol and growth hormone as described in man agree satisfactorily with those in other vertebrate species.