Electrodes were implanted into the rat hippocampus in both hemispheres for increasing periods of up to 60 days, and the effects of trauma and electrical stimulation of enzymes controlling cell metabolism in the region of implantation were examined and assessed in relation to studies on humans. In the unstimulated hippocampus as a control, enzyme changes were mainly confined to a narrow area of tissue damage surrounding the electrode. The enzyme changes in response to trauma varied widely; some enzymes controlling tissue respiration showed early and rapid changes, increasing in hyperactive, swollen glial cells and vascular endothelium and decreasing in nerve cells and neuropile. Acid phosphatase activity also increased rapidly in glial cells; other phosphate-releasing enzymes increased more gradually with time. A turning point in these chages was apparent between 25 and 40 days, followed by a reversion to more normal levels at 60 days. Electrical stimulation of the hippocampus in the contralateral hemisphere produced no detectable enzyme changes from those of the unstimulated hippocampus.