The Warburg method was used to study the action of adenosine on several phases of rat cerebral cortex metabolism, using cortex slices or homogenates. In the presence of exogenous glucose in vitro, oxygen consumption and lactate production are not affected by adenosine in sections. In vivo, there is an increase of oxygen consumption and of lactate production but which are not significant. Adenosine may activate metabolic pathways, since the observed metabolic changes remain constant during the period of activity of adenosine (30 to 60 min) and disappear concomitantly with adenosine. The action of adenosine is much more evident in sections from the brains of injected animals, where the increase of lactate production becomes significant. This suggests that in this case adenosine favors a better utilization of glycogen via an activation of adenylate cyclase. The increased activity of G-6-PDH was observed in vitro but was not significant in vivo. These observations were confirmed with homogenates from the in vivo series by the significant decrease of inorganic phosphate levels, consistent with an increased formation of nucleotide phosphates. The increased cerebral glucose concentration is perhaps a result of increased blood glucose levels, in turn resulting from the known depression of insulin release by adenosine, or from a preferential utilization of glycogen, resulting from the activation of adenylate cyclase.