Compelling evidence shows that intracellular free magnesium [Mg(2+)](i) may be a critical regulator of cell activity in eukaryotes. However, membrane transport mechanisms mediating Mg(2+) influx in mammalian cells are poorly understood. Here, we show that mechanosensitive (MS) cationic channels activated by stretch are permeable for Mg(2+) ions at different extracellular concentrations including physiological ones. Single-channel currents were recorded from cell-attached and inside-out patches on K562 leukaemia cells at various concentrations of MgCl(2) when Mg(2+) was the only available carrier of inward currents. At 2 mM Mg(2+), inward mechanogated currents representing Mg(2+) influx through MS channels corresponded to the unitary conductance of about 5 pS. At higher Mg(2+) levels, only slight increase of single-channel currents and conductance occurred, implying that Mg(2+) permeation through MS channels is characterized by strong saturation. At 20 and 90 mM Mg(2+), mean conductance values for inward currents carried by Mg(2+) were rather similar, being equal to 6.8 +/- 0.5 and 6.4 +/- 0.5 pS, respectively. The estimation of the channel-selective permeability according to constant field equation is obviously limited due to saturation effects. We conclude that the detection of single currents is the main evidence for Mg(2+) permeation through membrane channels activated by stretch. Our single-current measurements document Mg(2+) influx through MS channels in the plasma membrane of leukaemia cells.