Inorganic sulfate in cerebrospinal fluid (CSF) from 25 infants and children was measured by controlled-flow anion chromatography. The mean CSF concentration was 0.133 +/- 0.066 mumol/l for the group. Mean CSF sulfate is age dependent being 0.170 mmol/l in the newborn and 0.095 mmol/l (range: 0.059-0.165) in children over 3 years of age. The fall in CSF sulfate parallels a corresponding age-dependent change in serum sulfate. Accordingly, the CSF:serum ratio (0.334 +/- 0.019; mean +/- SE) remains constant in infants and children. The CSF:serum ratio departs significantly from that predicted by the Gibbs-Donnan equilibrium relation (predicted ratio, 1.21). CSF sulfate content appears to reflect mediated transport of sulfate out of CSF.