The incorporation of 14C-labelled leucine or phenylalanine into alkali-soluble protein was determined under in vitro conditions in aortic intima-media of normal and streptozotocin-diabetic rats. Two weeks after the induction of diabetes the incorporation of the amino acids into aortic protein was reduced. When determined after diabetes of one week's duration the leucine-14C incorporation was not significantly reduced, while after 5 weeks of diabetes it was severely impaired. After administration of insulin to diabetic rats in vivo for 2 weeks there was no difference in leucine-14C incorporation between normal and diabetic rats. Addition of insulin (0.1 U/ml) in vitro had no effect on the leucine-14C incorporation in either normal or diabetic aorta during incubation times of 3 or 6 h. Elevation of the glucose concentration in vitro from 5.6 to 22.2 mmol/l did not influence the leucine incorporation in diabetic aorta. Both the aortic wet weight and the aortic content of alkali-soluble protein were decreased after 5 weeks of diabetes. The decrease in the protein content of aorta of diabetic animals suggest that the protein synthesis is impaired in vivo.