Experimental data show that a significant improvement of growth rate and feed conversion rate of weanling pigs can be achieved by the inclusion of organic acids in the diet. These ergotropic effects have been mainly observed with formic, lactic, sorbic, fumaric, citric and malic acid as well as with different salts of formic acid. The lowering of dietary pH alone, with inorganic acids (o-phosphoric acid, HCl), however failed to show any nutritive efficacy. Studies on the mode of action of organic acids indicate that they cause a higher protein and energy digestibility and retention, an alteration of bacterial populations and metabolites in the gastrointestinal tract and possibly an effect on metabolism. It seems likely that the antimicrobial properties of organic acids and of their salts are of great importance for their beneficial effects in weanling pigs.