BACKGROUND: Protein fermentation products, especially ammonia, are implicated in the pathogenesis of certain diseases. AIM: To investigate the influence of lactulose and Saccharomyces boulardii cells on the composition of the intestinal microbiota and on the metabolic fate of ammonia by means of lactose-[(15)N, (15)N]-ureide. METHODS: An at random, placebo-controlled, crossover study was performed in 43 healthy volunteers to evaluate the influence of lactulose and/or S. boulardii cells either administered as a single dose or after a 4-week intake period. Urine and faeces were collected. All samples were analysed for (15)N-content by combustion-isotope ratio mass spectrometry. Real-time polymerase chain reaction was applied to determine the composition of the predominant faecal microbiota. RESULTS: A single administration of lactulose significantly decreased urinary (15)N-excretion in a dose-dependent way. After long-term administration of lactulose, a significant reduction of the urinary (15)N-excretion was observed, which was accompanied with a significant increase in the faecal (15)N-output, more specifically more (15)N was found in the bacterial fraction. A significant rise in the Bifidobacterium population was found after lactulose intake. No significant effects were observed after S. boulardii intake. CONCLUSION: Dietary addition of lactulose can exert a bifidogenic effect accompanied by a favourable effect on the colonic NH(3)-metabolism.