In monogastric animals magnesium is absorbed from the small and large intestine. In ruminants the forestomach system, in particular the rumen, is the most important site of magnesium absorption. Various mechanisms are involved in intestinal magnesium absorption (solvent drag, diffusion, carrier-mediated transport). In the large intestine and rumen an active transepithelial magnesium transport from the mucosal to the serosal side of the epithelium was recently demonstrated. Since in the large intestine and in the rumen, volatile fatty acids (VFA, mainly acetate, propionate, butyrate) deriving from fermentation of carbohydrates represent the major anions, the influence of VFA on magnesium absorption from these parts of the gut was recently investigated. VFA at physiological concentrations stimulated magnesium absorption in both cases. In the rat large intestine VFA enhanced only magnesium absorption by the distal colon, sodium and water absorption remaining unaffected. Both in sheep rumen and in the distal colon of the rat butyrate was most effective in this regard, followed in descending order by propionate and acetate. Sodium absorption by the rat proximal colon and caecum, and by the sheep rumen, was similarly enhanced by VFA. It has been suggested that the latter effect is due to the function of VFA as intracellular proton donators for the Na+/H+ exchanger located in the apical membrane of the epithelial cells. In analogy a Mg2+/H+ exchanger, located in the apical membrane of the epithelium in the distal colon and rumen, is fully consistent with the stimulatory effects of VFA on magnesium absorption at these sites.(ABSTRACT TRUNCATED AT 250 WORDS)