Five studies were conducted to elucidate a possible mode of action of dietary organic acids in weanling pigs (n=536). The effect of feeding 1.5% fumaric or citric acid on growth performance, intestinal digesta pH, chloride ion concentration (Cl-), VFA profile and microflora populations was studied. In study 1, feeding citric acid improved ADG and feed conversion (F:G), whereas fumaric acid only numerically improved ADG and F:G. No significant dietary effect on intestinal pH, Cl- or VFA profile was observed. In study 2, feeding organic acids with and without a microbial supplement (Lactobacillus acidophilus) resulted in no interactive effects, but fumaric acid and the microbial supplement each improved ADG and F:G. In study 3, pigs fed organic acids were killed at -2, 0, 3, 7, 14 and 21 d postweaning. Organic acids had no significant effect on intestinal pH, Cl-, VFA profile or microflora populations. Postweaning age affected intestinal digesta measurements in these ways: pH decreased; VFA concentrations increased; lactobacillus decreased to d 3 postweaning; E. coli increased to d 7 postweaning; and clostridia decreased to d 21 postweaning. In study 4, pigs were fed organic acids and orally challenged with 10¹⁰ enterotoxigenic E. coli (0157:K88) (ETEC) at d 3, 7 and 14 postweaning. Organic acids had no measurable effect on intestinal pH, Cl- or VFA profile. ETEC challenge increased jejunum lactic acid concentration, lower colon VFA concentration and severity of scouring. Postweaning age affected these intestinal digesta measurements: cecum pH decreased; jejunum Cl- increased; jejunum lactobacillus count decreased; lower colon lactobacillus count increased; and E. coli count decreased in all intestinal sections. In study 5, pigs were fed organic acids and challenge with ETEC on d 3 postweaning. Organic acids had no significant effect on severity of scouring or protection against reduced growth performance from the ETEC challenge. The ETEC challenge resulted in an increased severity of scouring and a reduction in ADG of 20% and F:G of 19%. In summary, feeding organic acids generally improved weanling pig growth performance and had little effect on intestinal digesta measurements. Postweaning age did have an effect on intestinal digesta measurements. The primary mechanism of action of dietary organic acids does not seem to be the lowering of intestinal pH or modifications to the intestinal microflora, but may act as an additional energy source. / Ph. D.