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In order to characterize Sterling's triiodothyronine (T3) mitochondrial receptor using photoaffinity labeling, we observed two specific T3-binding proteins in the inner membrane (28 kDa) and in the matrix (43 kDa) of rat liver mitochondria. Western blots and immunoprecipitation using antibodies raised against the T3-binding domain of the T3 nuclear receptor c-Erb A alpha 1 indicated that at least the 43-kDa protein was c-Erb A alpha 1-related. In addition, gel mobility shift assays demonstrated the occurrence of a c-Erb A alpha 1-related mitochondrial protein that specifically binds to a natural or a palindromic thyroid-responsive element. Moreover, this protein specifically binds to a direct repeat 2 sequence located in the D-loop of the mitochondrial genome. Furthermore, electron microscopy studies allowed the direct observation of a c-Erb A-related protein in mitochondria. Lastly, the relative amounts of the 43-kDa protein related to c-Erb A alpha 1 were in good correlation with the known mitochondrial mass in three typical tissues. Interestingly, expression of a truncated form of the c-Erb A alpha 1 nuclear receptor in CV1 cells was associated with a mitochondrial localization and a stimulation of mitochondrial activity. These results supply evidence of the localization of a member of the nuclear receptor superfamily in the mitochondrial matrix involved in the regulation of mitochondrial activity that could act as a mitochondrial T3-dependent transcription factor.