1. Distension of part of the uterus of the rat after parturition by an inert material ('Silgel') prevented involution, as measured by change in collagen content and weight in the distended part. The other parts of the uterus, those emptied at parturition, involuted at the normal rate. 2. Removal of the distending material four days after parturition was followed by involution (measured in the same way) in this part of the uterus. 3. Removal of all fetal material from one horn before the end of pregnancy (at day 19) was followed by involution, but at a slower rate than after normal term. Bilateral ovariectomy at the same time as removal of fetuses increased the rate of this involution, but it was still slower than at normal term. 4. Ovariectomy alone on day 19 had no effect on subsequent changes in the uterus; weight and collagen content then followed the normal course to term. 5. It is concluded that mechanical factors associated with the presence of material within the uterine cavity play a preponderant role in maintaining the uterine wall, though hormonal factors can affect its reaction to a lesser extent.
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