1 In order to elucidate the mechanism underlying the interactions between glucose and alloxan when competing for the sugar binding site of glucokinase from pancreatic B-cells or liver, the structural requirements of the enzyme for inhibition by alloxan and for protection by glucose were determined. 2 With a half-maximal inhibitory concentration of 5 /M, alloxan was the most potent pyrimidine derivative inhibitor of glucokinase. Uramil was a less potent enzyme inhibitor. A variety of other pyrimidine derivatives and related substances were ineffective. 3 Ninhydrin also inhibited glucokinase with a half-maximal inhibitory concentration of 5 gM. Isatin was a slightly less potent enzyme inhibitor. Several other indoline derivatives were ineffective. 4 Only glucose derivatives with a sufficiently bulky substituent in position C-2, such as the glucokinase substrates glucose and mannose and the inhibitors mannoheptulose, glucosamine, and Nacetylglucosamine, protected glucokinase against inhibition by alloxan by binding to the active site of the enzyme. Glucose epimers which differed in other positions did not protect the enzyme against alloxan inhibition. 5 DTT (dithiothreitol) protected glucokinase against inhibition by alloxan and reversed the inhibition of the enzyme induced by alloxan. Thus the mechanism of glucokinase inhibition by alloxan and other inhibitors, such as uramil and ninhydrin, is an oxidation of functionally essential SH groups of the enzyme, where the most reactive keto group of the inhibitor acts as the hydrogen acceptor. The protective action of glucose and several C-2 epimers demonstrates that these functionally essential SH groups are situated in the sugar binding site of the glucokinase. 6 The present results support our contention, that the pancreatic B-cell glucokinase is the major target mediating the inhibition of insulin secretion by alloxan.