It has been recognized that structural and functional defects of human mitochondrial NADH dehydrogenase (complex I) are involved in many diseases. Recently, sporadic Parkinson's disease and type 2 diabetes due to obesity are also considered to be related to complex I defects. Therefore, it seems important to develop therapies for complex I defects. Mitochondria of Saccharomyces cerevisiae lack complex I but instead have the rotenone-insensitive internal NADH dehydrogenase (Ndi1). The Ndi1 enzyme is composed of a single polypeptide and lacks proton pumping function. We have shown that the NDI1 gene encoding the Ndi1 protein can be functionally expressed in vitro. To extend the potential use of this enzyme to repair complex I deficiencies in vivo, we constructed a recombinant AAV vector carrying the NDI1 gene (rAAV-NDI1). We carried out injections of rAAV-NDI1 into muscles and brains of mice and rats. It turned out that the Ndi1 enzyme was actively expressed in muscles and brains. The expressed Ndi1 seemed to be localized to mitochondria in all tissues tested. Since the MPTP mouse model of Parkinson's disease is available, we have explored whether the Ndi1 enzyme protects against MPTP effect. The rAAV-NDI1 particles were unilaterally injected into the substantia nigra (SN) of mice. And then mice were administered with MPTP. A higher number of tyrosine hydroxylase (TH)-positive cells were seen in the SN of the injected side. At the level of striatum, it is evident that ipsilateral side exhibits distinctly higher TH density and higher dopamine content than contralateral side. The preliminary results strongly suggest that the Ndi1 enzyme may offer protection against neurodegeneration caused by complex I defects.