Abstractthe pyrimidines cytidine (as ctp) and uridine (which is converted to utp and then ctp) contribute to brain phosphatidylcholine and phosphatidylethanolamine synthesis via the kennedy pathway. their uptake into brain from the circulation is initiated by nucleoside transporters located at the blood-brain barrier (bbb), and the rate at which uptake occurs is a major factor determining phosphatide synthesis. two such transporters have been described: a low-affinity equilibrative system and a high-affinity concentrative system. it is unlikely that the low-affinity transporter contributes to brain uridine or cytidine uptake except when plasma concentrations of these compounds are increased several-fold experimentally.