Eight crossbred female pigs (40 kg) with cannulae placed in the terminal ileum were used to evaluate the effect of infusions of casein starch and casein plus starch on organic matter fermentation and microbial protein synthesis in the large intestine, and their effect on urinary urea and orotic acid excretion, and on nitrogen (N) retention. Infused casein and starch were both totally digested. Nitrogen retention was increased (P greater than 0.05) when casein was infused. Starch infusion resulted in an increase (P greater than 0.05) in fecal N in the form of total protein (amino acids). The high correlation (P greater than 0.01) between fecal total protein and RNA indicates that the increase in fecal N resulted from an increase in microbial protein synthesis. About 5.2 g of bacterial protein was synthesized per 100 g of cornstarch fermented in the large intestine. Casein infusion increase (P greater than 0.05) total urinary N. Differences between treatments for urinary N were entirely because of changes in urinary urea. Urinary ammonia and unaccounted N were not affected by treatments. Urinary orotic acid was a good indicator of the urea cycle activity because of its hig correlation (P greater than 0.0) with urinary urea. Plasma urea N concentration was increased (P greater than 0.05) only when casein plus starch was infused.