Postovulatory aging of oocytes significantly affects embryonic development. Also, altered Ca2+ oscillation patterns can be observed in fertilized, aged mouse oocytes. Because Ca2+ oscillations depend on Ca2+ release and reuptake in the endoplasmic reticulum, and the latter relies on ATP availability, we simultaneously measured changes in intracellular ATP concentration ([ATP]i) and Ca2+ oscillations in fresh and aged mouse oocytes. We continuously assessed changes in [ATP]i from intracellular free Mg2+ concentration measured by fluorescent dye Magnesium Green (MgG) while intracellular Ca2+ concentration ([Ca2+]i) was monitored by Fura-PE3. At fertilization, MgG fluorescence was transiently increased concomitant with the first transient elevation in [Ca2+]i, indicating a relative decrease in [ATP]i. In fresh oocytes, it was quickly followed by a significant decrease below baseline, indicating a relative increase in [ATP]i. In contrast, in aged oocytes, such a decrease in MgG fluorescence was not observed. In a separate experiment, ATP content in fresh and aged oocytes was determined in vitro by the luciferin-luciferase assay. Intracellular ATP contents measured in vitro were comparable in unfertilized fresh and aged oocytes. Intracellular ATP content at 5 h after fertilization was increased in both oocytes, where fresh oocytes showed a significantly higher intracellular value than aged oocytes. These findings suggest that aged mouse oocytes fail to readjust the level of intracellular ATP at fertilization. Relative deficiencies of ATP at fertilization might lead to an altered Ca2+ oscillation pattern and poor developmental potency, which is commonly noted in aged oocytes.