Increasing the dietary intake of K reduces the Mg absorption from the forestomachs of ruminants by decreasing the apical membrane potential and increasing the transepithelial potential difference (Vt). To evaluate genetic variations in the dependence of Mg absorption on K intake, we performed Ussing chamber experiments and studied the effect of Vt on 28Mg fluxes across rumen epithelium from different breeds of sheep. All breeds showed net absorption of Mg (JMgnet) in the absence of electrochemical gradients. JMgnet of Skudde was lower compared to Merino or Schwarzkopf sheep. Increasing Vt decreased mucosal to serosal Mg flux (JMgms) and increased serosal to mucosal Mg flux. The effect of Vt on JMgnet was almost identical in all breeds of sheep (−30 to −45% for a 25 mV increase in Vt). The effect of Vt on JMgms revealed a Vt-dependent and a Vt-independent component of transcellular Mg absorption in all breeds. Considering the driving forces for these two Mg transport mechanisms, the effect of K on Mg absorption will mainly depend on the feeding regimen, i.e. dietary K must have a relatively higher effect on Mg absorption at low Mg intake levels, which explains the correlation between pasture quality and tetany frequency as well as the missing effect of K on Mg status in some older studies.