SUMMARY Sodium and potassium levels were determined in the rat endometrium and uterine washings during normal and 'delayed' implantation. Both endometrium and uterine washings of normal rats differed from those of 'delayed' animals in their electrolyte concentrations. A dose of oestradiol dipropionate (1 μg/rat) capable of inducing implantation in 'delayed' rats (ovariectomized and maintained on progesterone) did not evoke any significant changes in electrolyte concentration of either the endometrium or uterine washings. These findings are discussed in the light of a hypothesis regarding delayed implantation in rats.