The dentate gyrus of adult rats was examined histochemically for cytochrome oxidase and lactate dehydrogenase activity after unilateral lesions of the entorhinal cortex. In normal animals, synaptic terminal fields of the perforant pathway from the entorhinal cortex show high levels of cytochrome oxidase activity (the outer two-thirds dentate molecular layer), whereas terminal zones of the commissural and associational fibers show high levels of lactate dehydrogenase activity (the inner one-third dentate molecular layer). Lesions of the entorhinal cortex result in a significant reduction in staining for cytochrome oxidase in the deafferented outer molecular layer of the dentate gyrus. The changes become prominent at 16-24 h after the lesion and persist until 90 days, the longest post-lesion survival time studied. In the non-deafferented inner zones ipsilateral to the lesion, there is an increase in staining for cytochrome oxidase and lactate dehydrogenase at 24 h post-lesion that disappears by days 2-4. From 8 to 90 days post-lesion, the band of high reactivity for lactate dehydrogenase in the inner molecular layer spreads approximately 40 μm into the overlying deafferented zone. This expansion parallels the expansion of the commissural and associational terminal fields into the adjacent deafferented molecular layer. Thus, lesion-induced synaptogenesis in the dentate gyrus is accompanied by a corresponding change in enzyme activity. The results indicate that the pattern of activity of enzymes involved in energy metabolism in the dentate gyrus depends on the distribution of pathway-specific synaptic input.