Alterations in concentration of glucose in the cerebrospinal fluid (CSF) are reported to be directly related to the level of blood glucose.*Pathologic conditions within the central nervous system may also change CSF levels, but will not be discussed here. It is difficult to evaluate previous results on relations of glucose in CSF and blood because of variations in experimental designs. Studies on blood are incomplete in some experiments, and total reducing substances or sugar rather than glucose were determined in most older reports. Reducing substances include compounds in addition to glucose and may be higher in blood than CSF.7,26Hypo- or hyperglycemia was usually produced by insulin or glucose administered by different routes. The effectiveness of insulin in producing hypoglycemia is variable and depends upon the tissue into which it is injected,35sex of patient,29presence of diabetes, binding to serum proteins,2and other factors.