Superoxide anion radical and superoxide dismutase, the enzyme responsible for dismutating it, are both present in the ovary and uterus of Mus musculus during early pregnancy. The detectable, stable levels of superoxide radical and the constant high levels of superoxide dismutase in the ovary during early pregnancy suggests that these may be involved in the regulation of extended luteal steroidogenesis for the maintenance of pregnancy. An inverse correlation between the levels of superoxide anion radical and superoxide dismutase in the uterus is shown. The high levels of superoxide anion radical in the uterus on the early morning of Day 5 of pregnancy point towards a probable role for this radical in the act of implantation and in mediating the increased vascular permeability at the initiation of implantation.