It has generally been assumed that free unconjugated bilirubin gains access to the brain because of its lipid solubility. However, no measurements of the bloodbrain barrier permeability to free bilirubin exist. The aim of these experiments was to determine the blood-brain barrier permeability in the rat to free bilirubin using singlepass (Oldendorf) andin Situperfusion (Takasato) techniques. Studies were performed on adult rats under sodium pentobarbitone anesthesia. [3H]bilirubin IX-α-ZZ (sp act 6.2 Ci/mmol) was synthesized by reduction of biliverdin with sodium boro-[3H]hydride. Blood-brain barrier permeability to albumin-bound and free bilirubin was determined using injectates/perfusates containing a molar excess of human serum albumin with or without the addition of the displacing agent sulphadimethoxine. For free bilirubin, the brain uptake index was 28.5 ± 9.3 (mean ± SD,n=18), and the permeability surface area product was 54.84 ± 36.38 × 10-4mL/s/g (mean ± SD,n= 13). These results demonstrate that thebehavior of free bilirubinin vivoin relation to the cerebral microvasculature corresponds to that of a “lipid soluble” molecule.