To determine the minimal amount of cytoplasm necessary to support normal development of mouse oocytes, mature unfertilised oocytes were reduced in size into approximately 1/2, 1/4 and 1/8 by removing the cytoplasm, then inseminated. More than 80% of 1/2-size oocyte were fertilised normally and almost all fertilised eggs developed to blastocysts. When transferred to foster females, 31% of the blastocysts developed into normal offspring. In contrast, 1/4- and 1/8-size oocytes, although they were penetrable by spermatozoa and extruded the second polar bodies, could not reach the 2-cell stage. In these oocytes, sperm nuclei did not develop into full-sized pronuclei. These results suggest that an oocyte can develop to full term after losing about half its cytoplasm, but not more.