Contrary to what is found in most vertebrates, the brains of certain turtle species maintain ATP levels and ion homeostasis and survive prolonged anoxia. The hypothesis tested here is that the release of adenosine and its binding to A1 receptors are essential for this anoxic tolerance. Studies were conducted in the isolated turtle cerebellum, which did release adenosine to the extracellular space during anoxia. When adenosine receptor antagonists [theophylline, 8-cyclopentyltheophylline (CPT), or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)] were added to the superfusate under control conditions, they had no effect on extracellular potassium ion activity ([K+]o). During anoxia, however, these antagonists provoked maximal efflux of K+ (anoxic depolarization). Anoxic depolarization occurred earlier during anoxia with theophylline (a nonspecific adenosine receptor antagonist) than with CPT or DPCPX, which specifically block A1 receptors. Therefore, adenosine release and effects mediated by A1 receptors are essential to anoxia tolerance in turtle brain.