The objective of our study was to investigate how Mg²⁺ enters mammalian cardiac cells. During this work we found evidence for a previously undescribed route for Mg²⁺ entry, and now provide a preliminary account of its properties. Changes in Mg²⁺ influx into rat ventricular myocytes were deduced from changes in intracellular ionised Mg²⁺ concentration ([fMg²⁺]_i) measured from the fluorescence of mag-fura-2 loaded into isolated cells. Superfusion of myocytes at 37 °C with Ca²⁺ free solutions with both reduced [Na⁺] but raised [Mg²⁺] caused myocytes to load with Mg²⁺. Uptake was seen with solutions containing 5 mM Mg²⁺ and 95 mM Na⁺, and increased linearly with increasing [Mg²⁺]_o or decreasing [Na⁺]_o. It was very sensitive to temperature (Q₁₀ > 9), was observed even in myocytes with very low Na⁺ contents, and stopped abruptly when external [Na⁺] was returned to normal. Uptake was greatly reduced by imipramine or KB-R7943 if these were added when [fMg²⁺]_i was close to the physiological level, but was unaffected if they were applied when [fMg²⁺]_i was above 2 mM. Uptake was also reduced by depolarising the membrane potential by increasing [K⁺]_o or voltage clamp to 0 mV. We suggest initial Mg²⁺ uptake may involve several transporters including reversed Na⁺/Mg²⁺ antiport and, depending on the exact conditions, reversed Na⁺/Ca²⁺ antiport. The ensuing rise of [fMg²⁺]_i, in conjunction with reduced [Na⁺], may then activate a new Mg²⁺ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB-R7943, but is inactivated by depolarisation.