The chromosomes of more than 3000 ovulated mouse oocytes from strains C3H/Han, NMRI/Han, and (101 x C3H)F1 have been analyzed after spontaneous and hormonally induced ovulation. No significant difference in the incidence of nondisjunction was observed among the three strains with or without hormonal pretreatment. The incidence of nondisjunction was estimated to be 0.47% in NMRI/Han, 0.62% in C3H/Han, and 0.81%) in (101 x C3H)F1. The incidence of chromosomal imbalance after the first meiotic division was slightly higher after adding the events following presegregation. Diploidy-spontaneous and hormonally induced-reached a significant level in NMRI/Han. This may be interpreted as a consequence of hormonal interference with a genetically defined malfunction of gene product(s) during the late phase of oogenesis.