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Distinct morphological changes were demonstrated in the contralateral hemisphere in the brains of Mongolian gerbils that were subjected to transient unilateral cerebral ischemia. The alterations were most obvious in a narrow region of the contralateral dentate gyrus (fascia dentata), where commissural inputs to the dentate gyrus are known to form synapses with the dentate granule cells. Electron microscopic examination revealed that these changes were caused by degenerative processes which took place in presynaptic terminals of the commissural inputs. An interesting fact was that the degenerated terminals were detected by light microscopy without the aid of special silver impregnation methods. After 1 week, these alterations almost disappeared, and after 3 months the dentate gyrus was undistinguishable from normal. These results strongly suggest that the changes were closely related to axonal degeneration and subsequent repair mechanisms of the brain. The present study indicates the importance of such anatomo-pathological study to delineate the effect of focal ischemia upon distant areas of the brain.