Young male rats were fed standard (normal protein content) or low-protein diets for 4 weeks. Protein-deficient rats were refed the standard diet for an other 1 2 or 4 weeks. Total cholesterol, triglycerides, phospholipids, total protein and hydroxyproline levels decreased in both aorta and plasma of rats receiving the low-protein diet. After the animals were refed the standard diet for 1 or 2 weeks, the lipid content of the arterial wall and plasma increased above basal values, returning to control values after the 4th week of standard-diet administration. Total protein levels increased slightly in plasma and arterial walls, reaching control values by the 4th week. The decreased hydroxyproline content in the arterial walls remained low after rats were refed the standard diet for 4 weeks.