To cover the Mg requirements of ruminants, large quantities of the ion are absorbed from the forestomachs. This corresponds to a high uptake capacity of ruminal epithelial cells (REC) for Mg. Investigations at the tissue and cell level led to the suggestion that Mg uptake involves both symport with anions and channel-mediated uptake. Various approaches were used to stimulate Mg current to levels detectable in patch-clamp measurements such as complete removal of cytosolic Mg and Ca, low cytosolic chloride, physiological cytosolic potassium concentration or replacement by choline or Cs. Externally, Mg was elevated to various levels (1,2,5,10, 30 and 70 mM). In choline chloride bath solution, no effect of Mg was observed on inward current at voltages down to ­120 mV. Effects on reversal potential were not significant. In more physiological NaCl solutions, Mg significantly reduced inward current. When Mg concentration was measured in intact REC cells by use of Mag-fura 2, a stepwise increase in extracellular Mg from 0 to 7.5 mM induced a 750 mM increase in cytosolic Mg, reaching a maximum level of 1.26 ± 0.1 mM (K(50)=1.2 mM). Uptake exceeded a speed of 1 mM/s which should lead to detectable currents at high driving forces for Mg. Saturation effects in a multi-ion channel or an electrogenic transporter should be considered when discussing the reasons for a failure to stimulate current to detectable levels in patch-clamp experiments on native REC cells.