The effects of proteins on plasma lipids depend on diet balance and the nature and amount of proteins in the diet but also on its fat and cholesterol content. Both the diet balance and study duration must be taken into account. Numerous investigations have been conducted with different methods, such as progressive decrease or increase of protein level in the diet. Because results differ according to the species, results for each species must be examined. Terpstra et al. (1983a, 1983b) and Sidranski (1990) have reviewed this subject.