Using the technique of vacuum infiltration, either a mixture of glucose plus fructose or a solution of sucrose alone was introduced into detached wheat leaves, and the rates of sucrose synthesis or hydrolysis from these infiltrated sugars were observed. It was found that these rates were not constant during 24 hr., but depended on the time of the day or night when the leaves were cut. Synthesis was high in leaves detached in the forenoon, before sunset, and in the first part of the night. Changes in the rate of hydrolysis were usually mirror images of those of synthesis. It has been concluded that in wheat leaves there exists a diurnal rhythm in their synthetic and hydrolytic potential and that the observed diurnal changes in the rates of sucrose synthesis and hydrolysis represent one of the manifestations of such a rhythm. Sodium fluoride, sodium cyanide, iodoacetate, and dinitrophenol always increased hydrolysis, and usually decreased synthesis. When glucose-1-phosphate, fructose-6-phosphate, and fructose-1, 6-diphosphate were substituted for the corresponding free sugar in a mixture of glucose plus fructose, a decrease in sucrose synthesis was observed. After 24 hr. of starvation the rate of sucrose synthesis declined, and eventually dropped practically to zero at the end of five days. In the same time there was a progressive increase in the rate of hydrolysis.