The ischymetric hypothesis of the control of food intake ( ischys = power in greek) suggests that the physiological stimulus of hunger is characterized by the decrease of the "m etabolisme de fond". In the same way, satiety would occur as long as the MF remains at a high level. The complex problems dealing with the continuous measurement of the MF in the free behaving Rat have been solved recently. As a consequence, it has been possible to test directly the ischymetric hypothesis by measuring the MF oscillations around six meals taken by the same Rat during a 24 hrs. experiment. The results showed that the meals are indeed preceded by a progressive decrease of MF and that the termination of meals coincides with a high metabolic rate. The rapid increase of the MF occurring immediately after the onset of the meals could reflect the oropancreatic reflex inducing the well known anticipatory release and utilization of the endogenous reserves. If anything, the respiratory quotient exhibited only small but not systematic variations during the pre-prandial periods. This observation is in agreement with the ischymetric hypothesis which proposes that the decrease of MF inducing the onset of a meal is due not only to the decrease of the glucose utilization but also to the concomitant decrease of the utilization of the three kinds of metabolic substrates.