Concentration of radioiodide by the uterus of the rat and the relationship to blastocyst implantation.

1. When pregnant rats are ovariectomized early in pregnancy and treated with progesterone a raised uterus-plasma concentration ratio (U/P ratio) for radio-iodide is observed at the usual stage of pregnancy (days 3, 4 and 5) but is not followed by implantation. When delayed implantation is induced by the administration of oestradiol it is not preceded by a rise in the U/P ratio for (131)I.2. Similar findings are obtained when implantation is delayed by concurrent lactation in rats mated at the post-partum oestrus.3. Ovariectomy early in pregnancy without progesterone treatment prevents the rise in U/P and oviduct-plasma (O/P) ratio for (131)I that normally occurs.4. Administration of progesterone to intact or ovariectomized non-pregnant rats produces a raised U/P and O/P ratio for (131)I; oestrogen may inhibit this response. The simplest explanation of these results is a direct action of progesterone on the uterus.5. By gross dissection it was possible to show that the region of high iodide concentration in the rat uterus is the endometrium.6. No evidence of a concentration of iodide in the uterus or oviduct of the pregnant rabbit was obtained.