A material which promotes ammonia metabolism in the body, as well as a material which has excellent ammonia metabolism improving effect and is effective for endurance enhancement and anti-fatigue are provided. An ammonia metabolism promoting agent, comprising catechins as an active ingredient. An agent for preventing or ameliorating hyperammonemia, hepatic encephalopathy or chronic fatigue syndrome, comprising catechins compound as an active ingredient. An ammonia metabolism promoting agent, an endurance enhancing agent and an anti-fatigue agent, comprising catechins, citrulline and arginine as active ingredients.