Transfer of bilirubin covalently bound to albumin (Alb·B) from blood into brain was examined in Gunn rats at postnatal day 3, 7, 14,28 or 70. The ratio of the 125I-Alb·B concentration in the brain to that in the plasma 2 h after the injection (RAB) was used as a criterion for Alb·B transfer. RAB in brain decreased with age until day 28. Significant correlations were indicated between 125I-Alb·B plasma levels and those in the brain at days 7 and 14, but not at day 28. These results suggested that 125I-Alb·B could transfer from the blood to the brain until day 14. Comparison of the relative amount of radioactivity accumulated in the brain by a 125I-Alb·B injection with that of 125I-albumin alone revealed higher radioactivity in the former than the latter only at day 14, suggesting a transient appearance of particular cellular elements with high affinity to bilirubin in the developmental stage of the brain.