The purpose of our study was to investigate the effect of hypermagnesemia on spinal metabolic rate. The 2-[14C]deoxyglucose technique was used to measure regional glucose utilization in the lumbar spinal cord of paralyzed, mechanically ventilated rats receiving 70% nitrous oxide and an intravenous infusion of either saline (n = 5) or magnesium sulfate (n = 5). Plasma magnesium concentrations were 6.75 +/- 0.5 and 0.9 +/- 0.5 mM (p less than 0.01) in hypermagnesemic and control rats, respectively. Hypermagnesemic rats were hypotensive (88 +/- 1 vs. 130 +/- 4 mm Hg, p less than 0.01) but blood pressure remained within the autoregulatory range. Glucose utilization was reduced 26-45% in spinal gray matter and 53-63% in spinal white matter during hypermagnesemia. We conclude that magnesium is a potent spinal metabolic depressant and that this action, which is unusually prominent in spinal white matter, is a plausible explanation for the recently reported beneficial effect of magnesium therapy during spinal cord ischemia.