Proline analogs are readily incorporated into collagen and noncollagen proteins. Since the imino acid content of collagen is greater than other proteins, it is suggested that the incorporation of a proline analog into cellular protein would have a maximal effect on collagen metabolism. Using a partially purified amino acyl tRNA synthetase preparation, various proline analogs were tested for their ability to inhibit Pro-tRNA synthesis. Amongst those tested, dehydroproline was the preferred inhibitor. Dehydroproline was also a substrate for amino acyl tRNA synthetase. When dehydroproline was added in vitro to membrane bound polysomes, the synthesis of collagenous proteins was preferentially inhibited. The addition of dehydroproline to mammalian cell cultures caused a marked reduction in prolyl hydroxylase activity. Under these conditions growth of cells, activities of lysyl; hydroxylase or lactic dehydrogenase were not affected. Reduction of prolyl hydroxylase activity by dehydroproline required protein synthesis. Removal of dehydroproline from the growth medium resulted in an increase in prolylhydroxylase activity. Hepatic fibrosis can be induced in rats by chronic administration of carbon tetrachloride. Under these conditions, the collagen content and prolyl hydroxylase activity of the liver is enhanced. Treatment of these fibrotic animals with dehydroproline results in a reduction of prolyl hydroxylase activity of the liver. A mechanism by which dehydroproline reduces prolyl hydroxylase activity will be discussed. Since prolyl hydroxylase plays a key role in the maturation and deposition of collagen, specific inhibitors of this enzyme are potentially useful in controlling collagen deposition in various pathological conditions.