Abstract: Study on protein requirement is the central research work in protein nutrition in ruminants and the exact measurement of endogenous nitrogen entering the rumen and efficiently utilized by microbial constitutes an important step for the estimation of true digestibility of dietary protein and protein requirements in ruminants. The objective of this article was to review studies on the sources of endogenous nitrogen, research methods of endogenous nitrogen recycling, endogenous urea nitrogen and total nitrogen entering the rumen of ruminants, and the proportion of nitrogen absorbed by microbe as reported in recent 30 years.