1. The protocollagen proline hydroxylase in mouse uterus was found to be similar to that in other animal sources in its subcellular distribution and cofactor requirements. 2. The activities of this enzyme in uterine tissue from non-pregnant mice were comparable with those in various embryonic tissues. 3. In the second half of pregnancy the protocollagen proline hydroxylase activity increased markedly. 4. After parturition the activity of this enzyme decreased rapidly, reaching normal non-pregnant values at 24h post partum. The results suggest a good correlation between the synthesis of collagen and the activity of protocollagen proline hydroxylase.