1. The isolated choroid plexus of the rabbit takes up 5-methyltetrahydrofolate from the incubation medium.
2. Other folate analogues (pteroylglutamic acid, methotrexate, 5-formyltetrahydrofolate = folinic acid) inhibited the uptake of 5-methyltetrahydrofolate.
3. The uptake of 5-methyltetrahydrofolate was inhibited by low temperature, anaerobic conditions and dinitrophenol.
4. The anticonvulsant drugs, diphenylhydantoin and phenobarbital, had no effect on 5-methyltetrahydrofolate uptake.
5. The inhibitory effect of pteroylglutamic acid on the uptake of 5-methyltetrahydrofolate by the choroid plexus may explain the effect of long-term folic acid therapy in aggravating vitamin B12 neuropathy in pernicious anaemia.