Magnesium absorption in ruminants is reduced by high potassium intakes (Fontenot et al., 1989), which might relate to a change in the transmural potential difference at the level of the rumen (Martens etal., 1987). In-vitro experiments with ruminal epithelium have shown that barium ions, which block potassium channels in cell membranes, alter the transepithelial potential difference when added to the mucosal side in the form of BaCl2, (Martens et al., 1987). Concentrations up to 4 mM Ba are used to block potassium channels in cell membranes. In the course of our studies on magnesium absorption in ruminants, we wished to alter the in-vivo transmural potential difference. To check whether BaCl2 would be a suitable tool, a test with two goats was carried out.