1. The effect of variation in arginine: lysine on the relative cholesterolaemic effects of dietary soya-bean protein and casein was studied. Male rats received semi-purified diets containing soya-bean protein isolate or casein supplemented respectively with varying amounts of lysine or arginine for 40 d and blood samples were taken after a 5 h fast. 2. Neither the addition of arginine to casein nor lysine to soya-bean protein modified the intrinsic effect of these proteins on serum cholesterol. 3. Serum triglyceride levels tended to rise with increasing amounts of lysine supplementation. The opposite trend was obtained with arginine supplementation. 4. Casein caused an increase in the concentration of serum insulin, but not glucagon. The glucagon level was increased proportionately with increasing amounts of arginine, while the addition of lysine showed no effect. The effects of added amino acids on serum insulin were inconclusive. 5. There was a parallel increase in serum apo E and glucagon in response to arginine supplementation, while lysine supplementation increased serum apo E. 6. Thus, arginine: lysine was more effective in regulating serum triglyceride than serum cholesterol. Insulin was associated with different effects of these proteins on serum lipids.