Rat thymocytes showed two Na⁺/Mg²⁺ exchangers with high- and low- affinities for external Na⁺ (Na⁺_o) at physiological internal Mg²⁺content. The total internal Mg²⁺ content (Mg²⁺_it) was enhanced by loading with MgCl₂ and the ionophore A-23187. Under these conditions, Na⁺/Mg²⁺ exchangers were dramatically stimulated by the Mg²⁺_it increase. Na⁺-induced Mg²⁺ effluxes were independent of Cl^-_o or H⁺. The Na⁺/Mg²⁺ exchangers, which we named HANao (high affinity for Na⁺_o) and LANao (low affinity for Na⁺_o), were dissected in Mg²⁺-loaded thymocytes according to their kinetics and stoichiometries. HANao, which showed an apparent dissociation constant for Na⁺_o (K_Na H) = 9.2 +/- 1.6 mmol l^-1 Na⁺_o and a maximal Na⁺ influx rate (V_Na(Na _H)_max) = 30.5 +/- 6.1 mmol (l cells)^-1 h^-1, was a 1Na⁺:1Mg²⁺ simultaneous antiporter insensitive to external magnesium (Mg²⁺_o) whereas that LANao, with K_{Na L} = 65.1 +/- 8.6 mmol l^-1 Na⁺ and a V_Na(Na _L)_max = 79.5 +/- 14.3 mmol (l cells)^-1 Na⁺ h^-1, was a 2Na⁺:1Mg²⁺ "ping-pong" antiporter which was strongly inhibited by Mg²⁺_o. At physiological concentration of Mg²⁺_o (1 mM), the Na⁺/Mg²⁺ exchange through the LANao was inhibited by ~50%. Amiloride (10^-4 M) inhibited at similar extent both Na⁺ and Mg²⁺ fluxes at high and at low Na⁺_o.