Fifty percent of New Zealand white rabbits became profoundly weak, had generalized seizures and died between 22 and 47 hours after an intravenous injection of 1000 IU/kg of L-asparaginase. The biochemical correlate of this syndrome is severe hypocalcemia associated with marked, single cell, oxyphilic necrosis in the parathyroid glands. Although survivors remained clinically well, they also developed hypocalcemia and parathyroid necrosis but to a lesser degree. Rabbits given an equivolumetric amount of saline did not develop alterations in any of these parameters. L-Asparaginase, therefore, exerts a direct toxic effect on the parathyroid glands of rabbits. The implications of this finding for man are briefly discussed.