1. It has been suggested that low levels of serum caeruloplasmin in Wilson's disease result from the failure to switch from a fetal to an adult mode of caeruloplasmin gene expression. To investigate postnatal expression of the caeruloplasmin gene, steady-state levels of caeruloplasmin messenger RNA in adult and neonatal rat liver were measured. 2. Copper parameters observed in neonatal rats were similar to those seen in Wilson's disease: hepatic copper concentration was significantly elevated (neonatal 164 ± 35 μg/g, adults 50 ± 8 μg/g, P < .001) and serum copper and caeruloplasmin levels were low (neonatal 0.5 ± 0.1 μg/ml, adults 1.3 ± 0.2 μg/ml, P < .001; neonatal 0.20 ± 0.04 arbitrary units, adults 0.69 ± 0.16 arbitrary units, P < .001), respectively. 3. Caeruloplasmin messenger RNA levels were analysed by Northern and dot blotting using a 12 P-labelled caeruloplasmin complementary DNA probe. A caeruloplasmin messenger RNA of approximately 4.4 kilobases was detected in both adult and neonatal rat liver, with no significant difference observed in steady-state levels. 4. A step subsequent to caeruloplasmin gene transcription must therefore be impaired in neonatal rats.