The postnatal development of bilirubin accumulation by the choroid plexus was studied in vitro. Choroid plexus of the fourth and lateral cerebral ventricles from adult or 1-, 7-, 15- or 21-day-old rabbits were incubated in a potassium-rich medium with 70 nmol/l of 3H-bilirubin for 5 min. The tissue/medium (T/M) ratio for 3H-bilirubin was considerable even 1 day post partum and increased during the next 2–3 weeks of life. In adults the 3H-bilirubin T/M ratio returned to the level of 1-day-old rabbits. The biliary excreted contrast agent iodipamide, added as a transport inhibitor, reduced the 3H-bilirubin T/M ratio in the choroid plexus, but to a relatively lesser extent in the 1-day-old rabbits than in the older ones.