The collagen content of the mouse uterus increases during pregnancy and rapidly decreases to the level of nulliparous mice within a few days post partum. Most of the hydroxyprohne in the uterine collagen had disappeared by day 3 post partum. During postpartum involution the hydroxyprohne concentration in both the serum and blood clot was not higher than in nulliparous mice. The total hydroxyprohne content in the liver, spleen and thymus was significantly increased during postpartum involution. These results indicate that materials derived from the breakdown of uterine collagen are rapidly removed from the blood stream by reticuloendothelial clearance.