The transfer of glucose, 2-deoxy-d-glucose, and fructose between blood and cerebrospinal fluid (CSF) of anesthetized dogs was studied, following either intravenous injection or intracisternal administration in artificial CSF. Glucose entry was not directly proportional to the concentration difference between blood and CSF. The CSF glucose concentration approached a maximum, despite increasing intravenous loads, suggesting saturation kinetics. Bidirectional molecular stereospecificity was demonstrated; the three hexoses enter and exit from CSF at increasing rates, fructose < glucose < 2-deoxyglucose. The slow entry of fructose is attributed to simple diffusion. The more rapid transfer of 2-deoxyglucose than of glucose suggests greater "affinity" of the former for the carrier. Bidirectional competitive inhibition between glucose and 2-deoxyglucose was demonstrated. Bidirectional countertransport ("uphill transport by counter flow") of glucose between the two compartments was shown. Glucose transfer into CSF was not affected by intravenous 30% urea. The data support the existence of a carrier transport system for glucose in membranes separating the two compartments.