It is demonstrated that adenylate kinase (AK) as well as NAD- and NADP-dependent glyceraldehyde-3-phosphate dehydrogenase (NAD- and NADP-GPD) display an endogenous rhythm in the time courses of their activities in constant conditions. The rhythm in AK activity has a period length of close to 30 h with two 15-h subpeaks in each 30-h period. The cycles in NAD- and NADP-GPD activity have a period length of 15 h and have an inverse phase relationship. The amplitude of the rhythm at the end of a dark period provides the base line for increase in enzyme activity in light after darkness. The increase in light is essentially the same regardless of the length of darkness. There is a differential effect of glucose feeding in light as compared to darkness. In darkness, glucose feeding increased NAD-GPD and AK activity but had no effect on NADP-GPD activity. In light, glucose caused an increase in NAD-GPD as well as AK activity over the Hoagland's controls but not in NADP-GPD, which was less than in the controls.The results are discussed in relation to a hypothesis assuming that endogenous rhythmicity might be due to the spatial separation of energy production and utilization in different cellular compartments with strong dependence on key coenzymes.