The presence of a number of interesting factors in kernicteric newborns with low serum bilirubin levels has lead us to study one important aspect, the reversible osmotic opening of the blood-brain barrier. Initially, an experimental hyperbilirubinemia was developed by the infusion of a crystalline bilirubin solution. Next, an increased serum osmolality state was generated by the injection of two different levels of urea (348.9 +/- 12.4 and 388.9 +/- 11 mOsm/kg). The bilirubin binding reserve capacity was not affected; however, we found a significant elevation in bilirubin concentrations in the homogenized brains of the hyperosmolality groups (2.92 +/- 0.54 and 3.4 +/- 0.43 mg/gr versus control of 2.43 +/- 0.63 mg/gr, F = 8.43, p < 0.01). Cerebral125 albumin was increased to similar proportions in those groups submitted to hyperosmolality.