A (25)Mg(2+)-operated hyper-activation of ATP synthesis has been investigated in mitochondria (Mt) isolated from iron-rich and iron-poor rat tissues: spleen, liver, skeletal muscle, myocardium, kidneys, brain. Both magnetic ((25)Mg) and non-magnetic ((24)Mg) magnesium isotopes were separately administered to estimate the degree of the ATP production related to the magnetic isotope effect (MIE) of (25)Mg(2+)as a function of the amount of Mt-endogenous iron ions. A strong but negative (r = -0.88) correlation between the (25)Mg-MIE degree and the Mt[Fe(2+)] values was found. The physical and biophysical mechanisms behind these phenomena, as well as the possible impact of these data on further biochemical and pharmacological studies involving (25)Mg-promoted nuclear spin selectivity in mitochondrial function, are under discussion.