The immunosuppressant drug cyclosporin A was first used in the clinic in 1978 in a kidney transplant surgery to prevent organ rejection. Its immunosuppressive activity is due to a Ca²⁺ -calmodulin-dependent binding of the cyclosporine-cyclophilin complex to calcineurin phosphatase. The inhibition of the calcineurin blocks the transcription of nuclear genes that depends on the nuclear factor for activation of T cells (NFTA) and prevents the production of cytokines, including IL-2. Recently, cyclosporin A has been used as a useful tool to protect membrane mitochondria against the deleterious effect of Ca²⁺ on membrane selective permeability. The closure of the non-specific pore by cyclosporin A appears to be related with the inhibition of the activity of mitochondrial cyclophilin, a peptidyl-prolil-cis-trans isomerase with a Ki of 2.5mM. Plausibly, the beneficial effect of cyclosporin on mitochondrial permeability transition underlies in the protective role of the drug on cell apoptosis and cell damage by ischemia-reperfusion.