Maturation of an immature oocyte into one capable of being fertilized involves tightly choreographed movements of chromosomes and organelles. The localization of mitochondria during maturation was studied in live mouse oocytes by confocal laser scanning microscopy (CLSM). Mitochondria were labeled with rhodamine 123 or Mitotracker (Molecular Probes, Eugene, OR) both of which are cell permeant and accumulate in mitochondria; acridine orange was used to mark chromatin. Prior to maturation, oocytes appeared to be radially symmetrical with no evident polarity; fully mature oocytes exhibited obvious polarity marked by the position of the metaphase II spindle in the cortex. CLSM revealed several interesting features of mitochondrial distribution: 1) A cortical clump of mitochondria was seen approximately 30-45 degrees to one side of the metaphase II spindle and marked the region of polar body I extrusion. 2) Large foci of mitochondria (7-14 microM) were frequently found around the central region of the mature oocyte, while the central region often exhibited markedly fewer mitochondria. 3) Small mitochondrial foci (3 microM) in the cortex and near the GV characterized several oocytes which failed to mature. 4) Non-spindle-associated mitochondria were not uniformly distributed in the mature oocyte but were concentrated in the hemisphere containing the metaphase II spindle. 5) The distal margins of this mitochondrial hemisphere were sharply demarcated at the cortex. These findings should help us understand organelle localization during mammalian oocyte maturation, and may give insights into possible causes of infertility and into early events of preimplantation development.