The presence of a silk thread in the lumen of the upper half of one horn of the uterus of the rat renders that horn sterile. The raised uterus-plasma concentration for radio-iodide that normally occurs on Day 3 of pregnancy is abolished by such a thread placed unilaterally; the control horn shows a normal response. The increase in the uterus-plasma ratio for 131I produced by the administration of progesterone to ovariectomized rats is significantly reduced, but not abolished, in a uterine horn carrying a thread. The inhibition by oestrogen of this response to progesterone is enhanced by the presence of a thread. Changes in the radio-iodide concentration ratio may provide a convenient model system for studies on the mechanism of action of an IUCD in the rat.