The hexokinase activity in the cerebral cortex of normal rabbits was measured by Longs method and the influences of fluoride, Ca ion, glutamic acid, glutamine, aspartic acid, asparagine, γ-aminobutyric acid and α-ketoglutaric acid upon the hexokinase activity were also evaluated. The results were as follows: 1) The fluoride shows the maximum inhibition to the hexokinase activity at 0.5M of its concentration, and the inhibition rather decreases at higher concentration. 2) Ca ion shows an increasing inhibition as its concentration increases. 3) When asparagine or glutamic acid is added, the activity is markedly accelerated. 4) γ-Aminobutyric acid accelerates slightly the activity of the cerebral hexokinase and markedly that of the heart muscle hexokinase. 5) α-Ketoglutaric acid markedly inhibits the activity.