Growing oocytes in vitro from the most immature stages until they are developmentally competent is a major goal of reproductive technology, requiring fundamental knowledge of metabolic processes. Carbohydrate metabolism and oxygen consumption have been analysed in a series of experiments designed to investigate important energy substrates for mouse oocytes and to reveal any qualitative or quantitative changes between the primordial and ovulatory follicle stages. Primordial follicles were incubated in groups in modified‐KSOM medium, whereas growing or ovulated oocytes were studied singly and, in both cases, the depletion or accumulation of metabolites in spent medium were analysed using ultramicrofluorometric assays. The rates of glucose (0.014 ± 0.006 pmol/hr) and pyruvate (0.028 ± 0.009 pmol/hr) consumption and l‐lactate (0.058 ± 0.023 pmol/hr) production by primordial follicles suggested that energy production was supported by a combination of metabolic pathways, including glycolysis. Pyruvate and oxygen consumption per oocyte increased two‐ and ninefold, respectively, between the primary and pre‐ovulatory stages (0.82 ± 0.1 and 1.67 ± 0.1 pmol pyruvate/hr, respectively and 1.4 ± 0.3 and 7 ± 0.6 pmol oxygen/hr) after which oxygen (12.7 ± 1.1 pmol/hr) utilisation nearly doubled. Oxygen consumption by fully grown oocytes was in excess of oxidation requirements for pyruvate. When pyruvate and oxygen consumption rates were normalised for oocyte cellular volume, which increased over 130‐fold during growth, oocyte metabolism was higher in primary follicles than at any subsequent stage, indicating that energy needs are greater during a developmental transition. To conclude, pyruvate and oxygen were consumed throughout oocyte development at increasing rates. When oocyte cellular volume was accounted for, oocytes from primary follicles displayed greatest metabolic rates. Mol. Reprod. Dev. 76: 231–238, 2009. © 2008 Wiley‐Liss, Inc.