Mitochondria play a vital role in the metabolism of energy-containing compounds in the oocyte cytoplasm to provide adenosine trisphosphate for fertilization and preimplantation embryo development. In this study, ratiometric confocal microscopy with the mitochondrion-specific membrane potential-sensitive fluorescence dye JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide) was used to measure the activity of mitochondria in human oocytes and developing preimplantation embryos. Mitochondria in oocytes and embryos were characterized by distinct localized aggregation patterns. These patterns however did not determine localized regions of heterogeneity in mitochondrial activity. Mitochondrial activity was analysed during oocyte maturation and after fertilization. The activity of mitochondria in fresh metaphase II oocytes was negatively correlated with maternal age. This trend continued when the activity of developing embryos was analysed. Mitochondrial activity was strongly correlated with the rate of embryo development on day 3 after fertilization, but not on day 2. Partial regression analysis showed that the rate of cleavage of preimplantation embryos was more highly correlated with embryo mitochondrial activity than maternal age. These data suggest that the efficiency of mitochondrial respiration in oocytes and preimplantation embryos is closely correlated with the programmed rate of embryo development, and suggest that maternal age further influences this factor. The loss of mitochondrial activity in oocytes obtained from ageing couples may therefore contribute to lower embryo development and pregnancy rates observed during cycles of IVF.