Numerous studies have reported comparisons of the nuclear-to-cytoplasmic (NC) ratio during mitosis. However, little information is known about how the pronuclear size is regulated and determined at the end of meiosis II in mammalian zygotes. The present study aims to analyze the NC ratio of female and male pronuclei, and also to compare the size of single pronuclei using photographs that were obtained during experiments to create chimeric hermaphrodites from 2-cell oocytes. The volume of both the female and the male pronucleus was found to correlate with the volume of the oocyte cytoplasm. The NC ratio of the male pronucleus was greater than that of the female pronucleus. The NC ratio of the average volume of the female and male pronuclei was greater than the NC ratio of the mononucleate oocytes. The occurrence of 1PN oocytes was significantly higher when the volume of cytoplasm was lower than the cut-off value. These results indicated that the NC ratio is retained during pronuclear formation. A higher NC ratio in male compared with the female pronucleus indicated structural and/or molecular difference between the two pronuclei. 1PN formation may occur when sperm enters close to the MII spindle.