SUMMARY: The uptake and intracellular localization of [6,7-3H]oestradiol has been studied in liver, kidney and mammary adenocarcinoma from female rats treated with dimethylbenzanthracene (DMBA). The tumour is similar to other oestrogen-responsive tissues in that the major oestrogen* within the tissue is oestradiol which is mainly associated with cell nuclei. The tumour has the ability to retain the oestrogen for at least 4 hr. Virtually no oestrogen is associated with the nuclei from liver and this result is discussed in relation to the intranuclear localization of oestrogen and the mechanism whereby oestrogen can enter the nuclei of some cells. A significant amount of oestradiol is associated with nuclei from kidney.