We recently reported that porcine plasma peptides affected serum lipids levels in rats. The present study was conducted to investigate the mechanism of improvement in lipid metabolism by porcine plasma peptides. Male Wistar rats were fed a 25% casein diet (casein group), or a porcine plasma peptide diet in which half of the casein was replaced by porcine plasma peptides (peptide group), or an amino acid diet in which half of the casein was replaced by an amino acid mixture identical to the amino acid composition of the porcine plasma peptide (amino acids group). All the animals were fed the experimental diets ad libitum for two weeks. Serum and liver lipids, urinary nitrogen and fecal cholesterol excretion were then determined. The serum total cholesterol level in the peptide group had a tendency to be lower than that in the casein group, and that in the amino acids group was decreased significantly compared with the casein group, but there was no significant difference between the peptide group and the amino acids group. The TBA values of both serum and liver in the peptide group were decreased significantly compared with the casein group. These results suggest that the serum total cholesterol level is affected by dietary amino acid composition, and that the TBA value is affected by the form of peptide.