This study was undertaken to determine where most of the uterine collagen is degraded during the postpartum involution. The greater part of uterine collagen, which was estimated as hydroxyproline, had disappeared by day 2 post partum. Colchicine, which decreases phagocytic activity, did not affect the uterine collagen degradation. Phagocytic activity in the post partum uterus did not appear until day 3 post partum. These results suggest that most of the uterine collagen is degraded by the extracellular but not by the intracellular pathway.