Uncoupling Protein l (UCPl) is an inner mito­ chondrial membrane protein, uniquely expressed in brown adipocytes, which uncouples the mitochondrial respiration impairing ATP production and energy efficiency. The aim of the present study was to express UCPl in liver mitochondria using a non-viral system in order to affect energy utilization. The effect of ectopic protein expression on liver energy metabolism, which was evaluated 42 h after DNA transfer, showed that mitochondria expressing UCPl presented decreased ATP production, lasted more time in membrane potential state 3, and consumed more molecular oxygen to produce the same amount of ATP than the control group. In summary, the successful functionality of the mitochondrial protein, UCPl, after hydrodynamic delivery is a novel and significant finding. This approach could be useful to ectopic­ ally express mitochondrial proteins and, in this particular case, to manage metabolic disorders related to energy efficiency and expenditure, such as obesity.