The aim of the present study was to report the levels of ascorbic acid in amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD) and the effectiveness of ascorbic acid homeostasis in the central nervous system. Plasma and CSF ascorbic acid levels were measured by high performance liquid chromatography in 19 ALS patients, 17 AD patients and 15 controls. No statistically significant difference was found between patients and controls. However, wide fluctuations of plasma concentrations were found to result in relatively stable CSF levels, by appropriate adjustments of CSF/plasma ratio. It appears that in normal subjects and in the disease under study, this ratio reflects the activity of the choroid plexus ascorbate transporter.