This study investigated different types of protein sources fed to dogs with Eck fistula. They used isocaloric diets with equivalent amounts of nitrogen derived from horse meat, fish fillets or skim milk. All animals had the same transient or chronic elevations of liver damage markers and decreased blood urea and serum albumin, with no differences in liver function tests. Survival periods were 26 days for horse meat, 54 days for fish filets and 68 days for milk. The first two groups showed symptoms of encephalopathy, whereas those in the milk group had no symptoms or less severe symptoms. There were differences in the amino acid residues, with more than double glycine, arginine, phenylalanine, and tryptophan residues in horse meat than in milk, suggesting that certain amino acids might have specific roles in the occurrence of symptoms.