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Magnesium transport across membranes was investigated in ferret red cells. Both Mg efflux and influx were measured. Mg transport was found to have the following properties. 1. Mg efflux from unloaded, untreated ferret red cells is 44 ± 2 μmol (1 cell)-1h-1 (mean ± S.E.M. n = 38). 2. Mg efflux is partially inhibited by amiloride, quinidine, quinine, imipramine and divalent cations. Efflux is stimulated by SITS (4-acetamdio-4'-isothiocyanato-stilbene-2,2'-disulfonic acid) and unaffected by vanadate. 3. Mg efflux is stimulated by the depletion of cell ATP. Reducing the cell ATP content reduces the Mg buffering capacity of the cells and thus increases the internal free Mg concentration ([Mg2+]i). This increase in [Mg2+]i may account for some of the stimulation of Mg efflux. 4. Reducing external Na ([Nao]) from 145 to 10 mM (Na replaced with choline choride) stimulates Mg efflux. However Mg efflux is inhibited when [Nao] is below 10 mM. Net uptake of Mg is measured when [Nao] is maintained below 1 mM even if the extracellular medium contains only contaminant Mg (Mg is transported against its electrochemical gradient). Changes in the membrane potential are not responsible for this net uptake. These results show that the system is capable of reversing direction and mediating active transport. 5. Net Mg uptake in media with low [Nao] is stimulated when [Mgo] is greater than 1 mM. The stimulated uptake is partially inhibited by amiloride, quinidine, imipramine, NEM (N-ethylmaleimide) and cobalt. Net uptake is stimulated by vanadate and inhibited by reducing cell ATP.