Proline hydroxylase activity was demonstrated in 15,000 x g supernate of homogenates of rabbit aorta. The characteristics of the rabbit aorta enzyme were compared to those of rat liver, a known source of proline hydroxylase activity. Ascorbate, Fe+2 and α-ketoglutarate were found to be necessary for maximum activity. Kinetic data indicate that the apparent Vmax and apparent Km for rabbit aorta enzyme and rat liver are similar in magnitude. These data confirm the presence of the collagen synthetic pathway in rabbit aortic tissue. Enzyme assays conducted on thoracic aorta indicated that proline hydroxylase activity is elevated six-fold in the presence of gross aortic plaquing. © 1969.