SummaryFolic acid activity was simultaneously assessed in serum and cerebrospinal fluid (CSF) in 51 aged patients (twice in 3 of them). Mean levels of folic acid activity in CSF and serum were 21.83±9.57 and 11.03±4.99 ng/ml respectively; mean ratio of the two values was 1.98; a positive correlation between serum and CSF folic acid concentrations was observed (P<0.01). Folic acid partition between serum and CSF varies according to the category of causal diseases. In patients without anemia and without major neurological syndrome the CSF/serum folic acid ratio is 1.85; it is 2.39 and 2.44 in ncuropsychiatric patients, and 1.55 in anemic individuals; mean CSF folic acid levels differ significantly in the last two groups of patients (P<0.05). Folic acid partition between body stores and nervous system is probably not automatically regulated in constant proportions, but is apparently governed by a complex mechanism.