In a two-factorial experiment on 96 young male rats, the effects of substituting 10% raw potato starch (PS), pectins (PEC), or cellulose (CEL) for corn starch (CS) were studied using an unsupplemented protein-free (PF) diet or a PF diet supplemented either with DL-methionine or urea. The pH and the short chain fatty acids (SCFA) content in caecal digesta, as well as caecal digesta and tissue weights were determined and used as the criteria of caecal fermentation intensity. Blood urea level, amount of N excreted via faeces and urine, DAPA content, and amino acid composition of faecal protein were analyzed as indices of protein metabolism. A 10-day adaptation period to the carbohydrates fed with the casein diet preceded the experimental period of feeding the respective carbohydrates with protein-free diets. Dietary carbohydrates significantly influenced total and individual SCFA content in caecal digesta, as well as other parameters related to the intensity of fermentation. Potato starch and pectins were more intensively fermented than cellulose. Supplementation of the PF diet with methionine and urea affected only caecal isobutyric and valeric acid content in a way dependent on the carbohydrates present in the diet. Carbohydrates significantly altered the routes of N excretion. Faecal excretion was increased by all carbohydrates studied compared to corn starch, pectins had the most marked effect. Urinary excretion was significantly increased by cellulose (as compared with the PEC and PS groups) and decreased by pectins as compared with all other groups. There was an interaction between the effects of carbohydrates and type of protein-free diet on faecal and urinary excretion. The sum of amino acids in faecal protein was the lowest on the PEC diet, but the amino acid composition expressed as a per cent of total amino acid content was similar in all groups. It can be concluded that dietary carbohydrates alter the excretion patterns of endogenous nitrogen in rats in different ways and that this effect is related to the intensity of their fermentation in the hind gut.