The author measured the activities of the membrane-bound enzyme, Na(+).K(+)-ATPase, and ATP as a part of an attempt to define the major cellular alterations in the initial 4-hour period of the feline spinal cord following experimental impact injury with 400gm-cm. The activities of Na(+).K(+)-ATPase were significantly decreased at post-injury 1-hour and between the 1-hour and 4-hour period. But for these same samples, the activities of Mg(2+)-ATPase did not change appreciably. Contents of ATP were significantly depleted in the same period. These changes suggested that this model of the spinal cord trauma results in a severe irrevirsible injury to the affected tissues.