There is increasing evidence that accumulation of redox-active iron in mitochondria leads to oxidative damage and contributes to various neurodegenerative diseases, such as Friedreich's ataxia and Parkinson's disease. In this work, we examined the existence of regulatory mechanisms for mitochondrial iron uptake and storage. To that end, we used rhodamine B- [(1,10-phenanthrolin-5-yl)amino carbonyl ] benzyl ester, a new fluorescent iron-sensitive probe that is targeted specifically to the mitochondrion. We found that extracellular iron was incorporated readily into mitochondria in an apparently saturable process. Moreover, the rate of iron incorporation responded to the Fe status of the cell, an indication that the mitochondrion actively regulates its iron content.