An improved understanding of nitrogen metabolism in the gastrointestinal tract of the pig is required in order to provide accurate estimates of nitrogen and amino acid (AA) bioavailability in feed ingredients and for adequate diet formulation. Research objectives were to estimate the extent of fermentative AA catabolism (FAAC) in the upper gut of pigs. Further objectives were to determine the impact of lower gut nitrogen absorption on measures of apparent ileal digestibility of AA and nitrogen, whole-body nitrogen balance, and urea kinetics in pigs fed a valine-limiting diet. It was determined that simple isotope dilution calculations are inappropriate for determining ileal ammonia flux and FAAC from a continuous infusion of labelled ammonia and urea. A static model with two-pools (blood urea and digesta ammonia) was then developed to determine possible value ranges for FAAC in the upper gut of pigs. Maximum estimated FAAC based on this model was lower when dietary protein content was decreased (P < 0.001). The model presented is limited to minimum and maximum estimates of FAAC due to the sampling and isotope infusion protocol used. Refinements to the model and experimental protocol could allow for more accurate estimates of FAAC. Infusion of casein or urea into the lower gut of pigs did not affect measures of apparent ileal digestibility of AA or nitrogen. These results further validate the methodology available for determination of ileal digestibility and the use of ileal digestibility to estimate bioavailability of AA and nitrogen. Infusion of casein or urea into the lower gut resulted in an increase in nitrogen balance and urea flux in growing pigs fed a valine-limiting diet. Nitrogen absorbed from the lower gut of pigs is likely in the form of ammonia which is converted to urea. Lower gut nitrogen can contribute to whole-body protein deposition via urea recycling and microbial AA production in the upper gut. Lower gut nitrogen absorption should be accounted for when estimating bioavailability of nitrogen in feeds and foods. / Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA); Ontario Pork; Canadian Swine Research and Development Cluster; Natural Sciences and Engineering Research Council of Canada (NSERC); Evonik Industries AG