The enzyme destorying FAD in spinach leaves were investigated. 1. FAD was hydrolyzed by nucleotide pyrophosphatase in spinach leaves. 2. The activity of the enzyme was considerably high in comparison with that of FAD pyrophosphorylase in green leaves. 3. The pH optimum was around 5.0 and the activity decreased remarkably at about pH 7.0, which was the pH optimum of FAD pyrophosphorylase. 4. The optimum temperature for the enzyme activity was approximately 60° when the reaction mixture was incubated for 20 min. 5. The Km value was 4×10^-5M and the relative substrate specificity of the enzyme for FAD was higher than those for NAD, NADP and ADP. 6. The hydrolysis of FAD was strongly inhibited by ATP, ADP, NAD and NADP occurring in green leaves about 10 times as much as flavins in concentration. 7. No hydrolysis of the tightly bound FAD with the apoprotein of glucose oxidase was recognized.