SUMMARY: The uptake of injected [6,7-3H]oestradiol by 9,10-dimethyl-1,2-benzanthracene-induced mammary tumours of the rat was investigated. Hormone-responsive adenomata differed from unresponsive adenomata in their ability to concentrate the hormone. The concentration of tritiated oestrogen present as free steroid in adenomata which failed to regress after ovariectomy was twice that found in muscle 20 min. after injection, and this ratio did not change significantly for 2 hr. after injection. Hormone-responsive adenomata concentrated the hormone to a much greater extent: the amount of tritiated oestrogen/mg. wet weight continued to rise for 100 min. after injection, when the concentration of radioactive steroid present in the tumours was 8–20 times that present in muscle. In this respect, the hormone-responsive tumours behaved in a similar way to the uterus and other target organs for oestrogen which have been investigated by other authors. Mixed adenomata, that is, those which contained both hormone-responsive and -unresponsive tissue, and fibroadenomata concentrated the injected oestradiol to an extent intermediate between the wholly responsive and wholly unresponsive adenomata.