The effect of a single large dose of parathyroid extract (PTE) in vivo on the synthesis of collagen by surviving bone fragments incubated in vitro has been examined at intervals ranging from 6 to 18 hr after injection. At all times studied the incorporation of radioactive proline label into both collagen proline and hydroxyproline was depressed. No selective depression of hydroxyproline formation in the final collagen was present. No accumulation of collagen precursors and no decrease in the specific activity of precursor hydroxyproline were demonstrable in crude cell fractions. The results are compatible with a feedback inhibition occurring at a late stage in the pathway of collagen biosynthesis. (Endocrinology74: 180, 1964)