1. Pregnant rats were injected with trace amounts of [125I] thyroxine ([125I]T4) or [125I]-tri-iodothyronine ([125I]T3).
2. Maternal blood, organs and foetuses were removed 1 and 4 h later, homogenized and trichloroacetic acid added to precipitate radio-labelled iodothyronines.
3. An hour after injecting [125I]thyroxine, 9–10 day old foetuses contained 21% of the radiolabel found in the same weight of plasma whereas by 13–14 days they contained 0.6%.
4. An hour after injection of [125I]tri-iodothyroninetri-iodothyronine, 9–10 day old foetuses contained 54% of the radioactivity present in the same weight of plasma.
5. When related to tissue protein, the radio-labelled iodothyronine content in 9–10 day old foetuses was comparable with that in maternal brain, heart or ovary.
6. De-iodination of [125I]thyroxine in 9–10 day old foetoplacental units was equal to that in maternal liver but greater than that in brain or heart.
7. It is concluded that substantial amounts of thyroxine and tri-iodothyronine enter the rat foetus in early pregnancy but thyroxine uptake may be minimal in later pregnancy.