An albumin-deficient and jaundiced strain of rats (AJR) was established by crossing Nagase analbuminemic rats (NAR) with jaundiced Gunn rats. AJR have no serum albumin and die with kernicterus within 3 weeks after birth. Their serum bilirubin level was 25 per cent (20 micrograms/ml) of that of Gunn rats at 1-2 weeks of age, while their brain bilirubin content was 1.2-2.7 times (4-5 micrograms/g brain) that of Gunn rats. Binding of bilirubin to NAR plasma proteins was examined. The bilirubin binding protein in NAR plasma was found to be lipoprotein, showing an association constant of 6.7 X 10(6) M-1. Bilirubin-transport into the brain of postnatal NAR was investigated by intravenous infusion of bilirubin with taurocholate. The brain bilirubin level of NAR infused with free bilirubin was 1.6 times that of normal rats. In NAR, the brain level on infusion of lipoprotein-bound bilirubin was similar to that on infusion of free bilirubin, but albumin-bound bilirubin scarcely entered the brain. These findings suggest that lipoprotein-bound bilirubin can diffuse across the blood-brain barrier into the brain.