Mouse eggs recovered from oviducts at one hourly intervals between 10 and 20 hours after administration of hCG were fixed, stained and then investigated the rate of in vitro fertilization and nuclear maturation. In case of out- bred ICR mice, ovulations were occured between 11 and 13 hours after hCG injection. The stages of in vitro maturation of eggs recovered from female mice at various times after hCG injection were metaphase I, anaphase I, telophase I and metaphase II. However the majority was metaphase I(17.6 to 44.4%) and metaphase II(42.9 to 80.0%) stage. When the eggs were inseminated with epididymal spermatozoa, the fertilization rate was declined as the egg recovery time after hCG administration was delayed. That is, the proportion of eggs undergoing fertilization became higher(68.1 to 77.4%) in the eggs at 12 to 15hr after injection of hCG than those(17.5 to 56.4) at 16 to 20 hr after injection of hCG. Also, when nuclear maturation of the unfertilized eggs were observed at 8 hours after insemination, the majority was in metaphase I and metaphase II and no anaphase I and telophase I were observed.