The energy metabolism ofChenopodium rubrumseedlings is analyzed with respect to endogenous rhythmic changes in adenine nucleotides. It is demonstrated that the pool sizes of ATP, ADP and AMP display endogenous rhythms. Following Atkinson's hypothesis of adenylate control of metabolism and determining the “energy charge”; of the seedlings, a circadian rhythm is observed which is shifted by 180° in relation to the circadian rhythm in betacyanin turnover. The energy charge concept is discussed in view of a working hypothesis which assumes that circadian rhythms evolved as an adaptation of living organisms to diurnal cycles of energy supply from the environment.