The earlier finding of a fat-feeding diurnal cycle in normal rats was further confirmed. Such a cycle was shown to be absent in hyperphagic hypothalamic rats. By use of a "C-labeled food eaten by rats either at night or day, it was directly shown that metabolites eaten and stored at night were mobilized during the subsequent day and were predominantly utilized during the hours preceding the onset of each meal. From various other results on the relationships between fat synthesis and overeating at night, and lipolysis and undereating in the day, it is concluded that a lipostatic mechanism modulates-both within the diurnal cycle and from day to day-the glucosensitive short-term control system of food intake. The disruption of this lipostatic mechanism in VMH-lesioned rats suggests that VMH nuclei are key sites for the neuroendocrine basis of the lipostatic control of food intake.