In kernicterus bilirubin appears at higher concentrations in the basal ganglia, the cerebellum, and the medulla. The mechanism for this distribution is unknown, but might involve differences in bilirubin entry among brain regions. We infused [3H]bilirubin (30 mg/kg over 5 min) intravenously into young Sprague-Dawley rats (n = 21, weight 144 +/- 10 g, mean +/- SD). Blood was sampled from the tail, and groups of rats were killed at 15, 30, 45, 60, and 75 min by intravenous pentobarbital. After in situ flushing of the brain vasculature, the brain was dissected into seven regions, each of which was weighed, dissolved in Soluene-350, and subsequently scintillation counted. Blood samples were analyzed for bilirubin (including specific activity), albumin, and blood gases. Brain bilirubin concentrations in each of the regions were calculated by comparing the radioactivity in that region with the specific activity of bilirubin. Using a curve-fitting program (EXPOFIT), the concentrations of bilirubin in each brain region at 5 min (i.e. the end of bilirubin infusion) could be estimated, and were as follows (nmol/g +/- SD): cortex 6.4 +/- 4.4; hippocampus 6.2 +/- 3.3; striatum 6.3 +/- 4.5; midbrain 7.3 +/- 4.4; hypothalamus 6.3 +/- 4.6; cerebellum 6.7 +/- 4.1, and medulla 7.0 +/- 4.0. There were no significant differences in the bilirubin concentrations between brain regions (F6,140 0.205). The mechanism for preferential localization of bilirubin to the basal ganglia probably does not involve the acute entry of bilirubin into brain.