Sixty-five male gerbils were exposed to 30 minutes of cerebral ischemia induced by a bilateral carotid artery occlusion. One group of 15 gerbils received a single injection of 25 microliter of 5 microM cyclohexyladenosine into the cerebral ventricle 15 minutes after release of the occlusion. Another group of 45 gerbils received a similar injection of the vehicle. Five days after ischemia, the hippocampal histology was examined under light microscopy. In the gerbils treated with the adenosine receptor agonist N-6-cyclohexyladenosine, the CA1 region of the hippocampus showed significant quantitative pyramidal cell preservation (p less than 0.01, Mann-Whitney U test). Qualitatively, substantial destruction of CA1 neurons was present in all hippocampi of the vehicle-injected gerbils. The CA1 neurons in the cyclohexyladenosine-treated gerbils did not differ from those seen in the five nonischemic controls. The precise mechanism of the protective action of cyclohexyladenosine is unknown, although it has been demonstrated that adenosine agonists reduce presynaptic glutamate release in vitro. It is possible that postischemic administration of cyclohexyladenosine decreases the release of this neurotransmitter in the intact brain as well. The concomitant reduction of the neurotoxic effect of glutamate may, therefore, result in better histologic preservation of the pyramidal cells in the postischemic CA1.