Silk threads placed in the uteri of ovariectomized mice increased cell proliferation in all tissues including regions of the uterus remote from the site of insertion. Many of the effects resembled those produced by oestrogens. An intra-uterine device (IUD) increased luminal and glandular mitosis and produced various degrees of luminal epithelial hyperplasia in untreated animals. In progesterone-treated mice bearing IUD's, luminal and stromal mitosis was increased. Epithelial morphology was not affected or luminal mitosis inhibited in oestrogen-treated animals with IUD's, but stromal and glandular mitosis was increased. After combined treatment with progesterone and oestrogen, stromal mitosis was suppressed at the contact site but was increased elsewhere. Both oestrogen and progesterone suppressed the luminal leucocytosis induced by the IUD. Despite this, the IUD prevented complete progestational differentiation of the luminal epithelium and closure of the lumen. The degree to which IUD-induced abnormalities were observed depended on the hormonal status of the animal at the time of sampling.