The transfer of thyroxine from the mother to the young of the marsupials, the bandicoot (Isoodon macrourus) and the possum (Trichosurus vulpecula).

1. The young of the bandicoot and possum are of similar weight at birth (200-250 mg) but by day 50 post-partum have weights of 100 and 20 g, respectively. The aim of this study was to determine whether the disparate growth rate was associated with a difference in the transfer of thyroxine from mother to young. 2. Radioactive thyroxine was injected intramuscularly into the lactating female and 24 hr later a blood sample was obtained from the mother and the pouch young removed. The amount of radioactive thyroxine remaining in the blood and within the young was determined. 3. The data obtained indicated that milk is a source of thyroid hormones in early pouch life. 4. The hypothesis that greater quantities of thyroxine are transferred in the milk of bandicoots than that of brushtail possums is not supported.