Authors analyzed total and free bilirubin concentrations (TB and FB) and free fatty acid levels (FFA) (by enzymatic methods) in the serum (S) and cerebrospinal fluid (CSF) of 26 sick newborn infants. Relationship between these three biochemical parameters in both S and CSF and also between these latter 15 studies FFA serum concentration of neonates (n: 9) with TB less than 2 mg/dl - 70.15 (15.63) (+/- SEM) mg/l - was not statistically different from those calculated in CSF - 67.45 (13.80) mg/l - (p: NS): while newborn infants (n: 17) with TB greater than 2 mg/dl had FFA serum levels - 152.75 (22.84) less than 0.002). There was a positive correlation between TB and FFA in S (n: 26, r: 0.52, p less than 0.01). There was no correlation of the FFA levels between S and CSF (n: 26, r: -0.02, p: NS). In CSF, FFA concentrations were correlated with albumin levels (n: 26, r: 0.67, p less than 0.005) and white cell recound (n: 26, r: 0.53, p less than 0.01). Authors discuss results and conclude that: a) bilirubin interferes with FFA metabolism, probably as the result of a toxic effect on extraneuronal tissue; b) an increase of FFA serum concentrations does not cause them to rise in the CSF by transport across blood cerebrospinal fluid barrier; and c) although, the source of the FFA in CSF is varied (from serum or loss of myelin, for instance, as a consequence of an asphyxial insult, or devitalized white cells) their increased concentrations are correlated with enhanced albumin levels and white cell recounts.