Relevant research on ammonia production, ammonia absorption, and urea recycling in ruminants was reviewed. Ammonia production and utilization in the rumen and post-ruminal digestive tracts are described in detail. Absorption of ammonia into portal-drained viscera, ammonia detoxification and urea synthesis in the liver, and urea degradation in the gastrointestinal tract are then discussed. The factors of affecting urea recycling and its pathways are also analysed. Suggested future research should focus on urea recycling dynamics and improvement of protein conversion efficiency, and on the development of an integrated mechanistic model to describe the digestion and metabolism of nitrogen-containing compounds in ruminants fed practical diets.