Adenosine A1 receptors, as demonstrated by [3H]cyclohexyladenosine (CHA) binding to cryostat sections of the brain, were studied utilizing quantitative autoradiographic techniques. A brief period of global CNS anoxia resulted in the rapid and persistent down regulation of [3H]CHA binding sites in the hippocampus but not in the neocortex or striatum. The density of adenosine A1 receptors in a given brain region has previously been shown to be a critical factor in determining the strength of the inhibitory action of adenosine. Since the down regulation of these sites is correlated temporally with the onset of hyperactivity following transient anoxia, it is suggested that a reduction in the strength of the neuromodulatory action of adenosine contributes to the postanoxic hyperactivity of CA1 pyramidal cells and perhaps to their selective vulnerability.