The incidence of chromosome anomalies was studied in fertilized oocytes in two groups of hybrid mice in which superovulation was induced by gonadotrophins and growth hormone-releasing factor (GRF) supplementation or gonadotrophins alone (controls). The rate of fertilization was significantly higher among GRF-treated females than among controls (74.1 versus 84.7%; P < 0.013). Cytogenetic data were obtained in 262 fertilized oocytes (89 from control females and 173 from GRF-treated females). The frequency of aneuploidy, calculated as twice the frequency of hyperhaploidy was 2.31% in GRF-treated females and 2.24% in controls (NS). The use of GRF to treat female mice did not adversely affect the maturation process of oocytes nor did it induce an increased frequency of aneuploidy.