We have studied the effect of moderately impaired maternal insulin secretion on oocyte chromosomal constitution, fertilization and zygote DNA synthesis. Female mice were injected with a single dose of streptozotocin (65 mg/kg) 14 days before fertilization/ovulation. Zygotes/oocytes were recovered from control and subdiabetic mice on day 1 of pregnancy. Compared with control animals, subdiabetic females showed a significant difference in the proportion of zygotes/oocytes. The subdiabetic mothers had a lower percentage of zygotes and a higher percentage of unfertilized and degenerated oocytes in comparison with control animals. The investigation of [3H]thymidine incorporation did not show any influence of the maternal subdiabetes on the initial zygote DNA synthesis. An analysis of the ovulated oocytes at the metaphase II stage isolated from subdiabetic mice did not reveal increased chromosomal anomalies in comparison with the controls. Control and subdiabetic mothers had a similar percentage of oocytes with a normal haploid set of chromosomes, and the incidence of aneuploidy/diploidy did not differ significantly. These observations suggest that insulin changes in subdiabetic mothers had a deleterious influence on oocyte fertilization in mice, but apparently they did not have any effect on the nuclear events.