The effect of an intrauterine thread on tissue changes in the endometrium of the pseudopregnant rat.

1. Changes in the fresh weight of uterine horns bearing unilateral intrauterine threads have been measured during early pseudopregnancy in the rat.2. Such a thread, which acts as an intrauterine contraceptive device, causes an increase in weight in the horn which carries it, compared to the untreated contralateral horn, on days 2, 4, 5 and 7 of pseudopregnancy. This increase is observed when the device has been present either for 25 or for 45 days.3. The fresh and dry weights of samples of endometrial tissue together with the concentration of chloride and potassium in these samples have also been determined, both in control horns and in horns which have contained a device for 45 days.4. The fresh and dry weights of samples from the device side are greater than the control. However, the weights of the endometrial samples recovered showed considerable variation, particularly in the device side. The weight of water per unit dry weight of endometrial tissue remains constant in the control side but falls steadily from day 2 to day 7 in the device side. The extracellular fluid space, as measured by the weight of chloride per unit dry weight, follows a similar pattern in each side. The weight of potassium per unit dry weight of control tissue falls on days 4 and 5 but rises again on day 7. By contrast, the potassium content of endometrial tissue from the device side falls steadily, reaching its lowest value on day 7.5. These results suggest that the presence of an intrauterine thread alters the composition of the connective tissue matrix of the uterine endometrium. It is likely that these changes are partially responsible for the contraceptive action of such a foreign body.