In the lower half of the secondary pulvinus which is the motor organ for the circadian leaf movement, the interruption of light leads to either a very rapid or a very slow decay in the glucose content. This effect depends on the time when the dark treatment is given during the circadian cycle. Accordingly, the catabolism of glucose in this part of the pulvinus is high when the energy requirement for the rhythmic leaf movement is high. We suggest that the glucose metabolism, and thus most likely the general carbohydrate metabolism, is under the control of the circadian biological dock.