The effects of glucose and oxygen on cultures at the stationary phase were also studied. Table 1 summarizes the results. As also reported by Manners & Ryley (1952), Tetruhymenu can store large quantities of glycogen that serve as an energy source in anaerobic cells. The glycogenic capacity is lower in cells from shaken cultures, whereas cells from glucose-containing cultures cannot convert acetate into glycogen (Hogg & Kornberg, 1963; Levy & Hunt, 1967). When cells that had been grown in glucose-free medium until the stationary phase were transferred to a glucose-supplemented medium, they showed increasing amounts of glycogen synthesis during the incubation time of 180niin. The amounts in static cultures were always higher than in shaken cultures. No remarkable changes in the activities of synthase or phosphorylase could be observed during synthesis of glycogen after addition of glucose. Also in these experiments the total and a forms of the glycogen-metabolizing enzymes did not show differences. From these studies it is clear that glycogen synthase a and phosphorylase a activities are independent of glucose concentration or incubation conditions. Furthermore there was no relationship between the amounts of glycogen deposited and the synthase activity in the u form at the stationary phase. It may be that the glycogen synthesis in Tetruhymenu pyriforrnis is regulated by intracellular metabolites. A role for peroxisomes in the regulation of glycolysis in this protozoan has also been suggested (de Duve & Baudhuin, 1966). Further investigation to clarify the mechanism of control involved in glycogen metabolism in these organisms is needed.