Introduction During the time of weaning the incompletely developed digestive system of weaning piglets is stressed by the feed, leading to a higher incidence of diarrhoea. We have investigated the impact of prophylactic feeding of dried blueberry and/or dried peat on the incidence of diarrhoea in weaning piglets. The antidiarrhoeal properties of blueberry are attributed to their phenolic substances such as tannins and tannin precursors. Peat seems to act in an auxiliary manner due to its content of humic acids, which have antimicrobial and astringent properties. Materials and Methods For a period of 14 days, four different rations were fed to 159 weaned piglets. The rations were either control diet (A), or enriched with 5 % of dried peat (B), 5 % of dried blueberry (C) or 2.5 % of peat and 2.5 `)/0 of blueberry (D). The faeces of each piglet were assessed once a day on days 5, 6, 7, 8 and 9 of the trial, the scores summarized and the individual antibiotic treatments of diarrhoea documented. Results With a sum of 44 for group C and of 45 for group D, the scores of the faeces were lower for the rations comprising blueberry than for the control group A (Sigma = 89) or the peat group B (Sigma = 106). There was a significant difference between groups B and C. Five and six piglets of groups C and D were therapeutically treated with an antibiotic, compared to twelve piglets of each of groups A and B. Neither supplement had a statistically significant impact on parameters such as body mass, daily weight gain and feed consumption or feed conversion. Conclusion Supplementing the diet of weaning pigs with peat had no effect on the incidence of diarrhoea. Feeding a ration containing 2.5 % and 5 % dried blueberry resulted in lower, but statistically not significantly different, scores for faeces than for the control group and in fewer, but statistically not significantly different, antibiotic treatments. This slight effect and the relatively high price of the ingredient do not justify the application of blueberry as a prophylaxis against diarrhoea in practice.