The present study investigated the stimulatory effects of iron (or ascorbate) on cyclosporine-induced kidney mitochondrial damage. Damaging effect of 50 μM cyclosporine plus 20 μM Fe2⁢ on mitochondrial lipids and proteins of rat kidney and hyaluronic acid was greater than the summation of oxidizing action of each compound alone, except sulfhydryl oxidation. Cyclosporine and 100 μM ascorbate showed an enhanced damaging effect on lipids but not on proteins. The peroxidative action of cyclosporine on lipids was enhanced with increasing concentrations of Fe2⁢. Ferric ion (20 μM) also interacted with cyclosporine to stimulate lipid peroxidation. Damaging action of cyclosporine on mitochondrial lipids was enhanced by ascorbate (100 μM and 1 mM). Iron chelators, DTPA and EDTA, attenuated carbonyl formation induced by cyclosporine plus ascorbate. Cyclosporine (100 μM) and 50 μM Fe2⁢ (or 100 μM ascorbate) synergistically stimulated degradation of 2-α deoxyribose. Cyclosporine (1 to 100 μM) reduced ferric ion in a dose dependent manner, which is much less than ascorbate action. Addition of Fe2⁢ caused a change in absorbance spectrum of cyclosporine in 230∼350 nm of wavelengths. The results show that cyclosporine plus iron (or ascorbate) exerts an enhanced damaging effect on kidney mitochondria. Iron and ascorbate appear to promote the nephrotoxicity induced by cyclosporine.