A novel mitochondrial localizing ruthenium(II) peptide conjugate capable of monitoring dynamic changes in local O2 concentrations within living cells is presented. The complex is comprised of luminescent dinuclear ruthenium(II) polypyridyl complex bridged across a single mitochondrial penetrating peptide, FrFKFrFK-CONH2 (r = d-arginine). The membrane permeability and selective uptake of the peptide conjugate at the mitochondria of mammalian cells was demonstrated using confocal microscopy. Dye co-localization studies confirmed very precise localization and preconcentration of the probe at the mitochondria. This precision permitted collection of luminescent lifetime images of the probe, without the need for co-localizing dye and permitted semiquantitative determination of oxygen concentration at the mitochondria using calibration curves collected at 37 degrees C for the peptide conjugate in PBS buffer. Using Antimycin A the ability of the probe to respond dynamically to changing O-2 concentrations within live HeLa cells was demonstrated. Furthermore, based on lifetime data it was evident that the probe also responds to elevated reactive oxygen species (ROS) levels within the mitochondria, where the greater quenching capacity of these species led to luminescent lifetimes of the probe at longer Antimycin A incubation times which lay outside of the O-2 concentration range. Although both the dinuclear complex and a mononuclear analogue conjugated to an octaarginine peptide sequence exhibited some cytotoxicity over 24 h, cells were tolerant of the probes over periods of 4 to 6 h which facilitated imaging. These metal-peptide conjugated probes offer a valuable opportunity for following dynamic changes to mitochondrial function which should be of use across domains in which the metabolic activity of live cells are of interest from molecular biology and drug discovery.