Experiments were conducted to establish the impact of diets designed to induce either low or high endogenous gut protein losses (ENL) on protein synthesis rates and energy expenditure in the visceral organs of growing pigs. First, conditions were established for converting lysine in practical feedstuffs into homoarginine (HA). This technique was then used to determine ENL in pigs fed casein-cornstarch (CC), barley (B), canola meal (CM) or a mixture of barley and canola meal (BCM) based diets. Apparent and true ileal amino acid digestibilities were also determined. Diets based on B, CM and BCM induced significantly more ENL than the CC-based diet. True ileal amino acid digestibilities in the BCM diet seemed more additive compared to apparent digestibilities. A preliminary study was conducted to confirm that a flooding-dose of phenylalanine 'per se' has no significant impact on the metabolic status of the pig. This flooding-dose technique was then used to measure protein synthesis rates in visceral organs of growing pigs fed either the CC- or the BCM-diet. The BCM diet increased daily protein synthesis rates in the colon but not in the small intestine. This finding suggests that the increase in ENL observed when feeding the BCM-based diet is due to reduced re-absorption of endogenous nitrogen rather than an increase in the secretion of endogenous nitrogen into the gut lumen. A further study was conducted to determine diet effect on organ mass and in vitro oxygen consumption in some visceral organs. In this study, the CC and BCM diets were evaluated as well as a BCM based diet with 30% added alfalfa meal (BCM-ALF). The results showed that in vitro oxygen consumption per g of tissue is not affected by diet type. The BCM and BCM-ALF diets increased organ mass compared to the CC diet and because of this effect, total energy expenditure in the visceral organs of BCM and BCM-ALF pigs was higher than in the CC-fed pigs. These findings suggest that increased ENL as a result of feeding a BCM diet compared to a CC based diet, is associated with increased energy expenditure in the hind gut. The latter is largely attributed to diet effects on organ size. It is suggested that further studies be conducted to evaluate relationships between ENL, size and energy expenditure in visceral organs when specific dietary components (e.g. feed enzymes, antinutritional factor etc.) are manipulated.