During the past two decades endogenous gut N losses (ENL) at the distal ileum in the growing pig have received considerable attention in swine nutrition research. Estimates of ENL are important for determining true ileal N and amino acid digestibilities and for identifying means to improve the efficiency of N and energy utilization in growing pigs. Endogenous secretions originate from various sources including saliva, pancreatic secretions, bile, sloughed off epithelial cells, serum albumin and mucin. It has been estimated that 70 to 80% of endogenous N secretions are digested and re-absorbed. Therefore, ENL represents only a fraction of total endogenous N secreted into the gut. Increased ENL are likely associated with elevated rates of gut protein synthesis. This is bound to increase maintenance energy and amino acid requirements of pigs. Traditionally, ENL were determined by feeding protein-free diets or by the regression method. Various alternative techniques (15N-isotope dilution technique, homoarginine technique, enzymatically hydrolysed casein method) are now available to estimate the ENL in pigs fed protein-containing diets. Each of these techniques has some limitations and all require different assumptions. Results obtained with these alternative techniques indicate that the net ENL losses are much higher and more variable than previously estimated, and that they are affected both by animal and dietary factors. Recent estimates of ENL losses vary between 1.8 and 8.3 g kg−1 dry matter intake. The main factors (feed intake, body weight, content of anti-nutritional factors, fibres and [digestible] protein) that affect ENL and approaches (plant breeding and selection, ingredient processing and use of exogenous enzymes) to reduce ENL are discussed in this review. In addition, the metabolic costs associated with ENL are estimated. Key words: Endogenous nitrogen losses, growing pigs, metabolic cost, measuring techniques