Experiments were conducted to determine the effect of different levels of fumaric acid supplementation to diets with a low or high buffer capacity on the concentrations of microbial metabolites and lipopolysaccharides, as an indicator of gram negative bacteria in ileal digesta of young pigs. In two experiments, 12 pigs each were weaned at 14 d of age and fitted with a simple T-cannula at the distal ileum between 15 and 17 d of age. In experiment 1, the pigs were fed wheat-soybean meal diets without or with inclusion of 1, 2 or 3% fumaric acid according to a balanced two-period changeover design. In experiment 2, the same diets were fed, except that the dietary buffering capacity was increased by inclusion of 3% sodium bicarbonate to all diets. The pigs were fed three times daily, equal amounts at 8 h intervals. The diets were supplied at a rate of 5% (wt/wt) of body weight. The inclusion of fumaric acid to the diet with a low buffering capacity (exp. 1) decreased (P < 0.05) the concentrations of lactic acid, ammonia, spermidine and lipopolysaccharides in ileal digesta. Supplementation of fumaric acid to a diet with a high buffering capacity (exp. 2) did not affect (P > 0.05) the concentrations of fermentation products in ileal digesta, but there was a decrease (P < 0.05) in the concentration of lipopolysaccharides. Furthermore, in both experiments, the concentration of most fermentation products decreased (P < 0.05) with increasing age after weaning. These results give further evidence that supplementation of fumaric acid to diets for young pigs during the first 3 - 4 wk after weaning reduces the metabolic activity and the concentrations of bacteria in the gastrointestinal tract. The magnitude of this effect, however, is dependent on the buffering capacity and the inclusion level of fumaric acid in the diets.