The use of cortical slices in the study of cerebral metabolism permits a wide range of experimentation, and yields information not readily obtainable from in situ preparations. The incubation media principally employed for such in vitro systems have been variations of the Krebs‐Ringer solutions (Krebs and Henseleit, 1932; Mcilwain,1959a; Heald, 1960). Although capable of supporting adequate respiration of the separated tissue, such media do not sustain all metabolic responses comparable to in vivo conditions (Mcilwain, 1959b). This paper is concerned with the effects of added creatine and ethanolamine on the metabolism of their phosphates in rabbit cerebral cortex slices in vitro. These compounds were chosen because of the sensitivity of phosphoryl creatine to metabolic deficiency as well as its important involvement in energy metabolism, and because of the rather wide‐spread presence of phosphoryl ethanolamine in animal tissues (Shaw, 1955; Long, 1961; Cohen and Lin, 1962). Some experiments were also done with slices from very young rabbits for comparative purposes.