Summary. The vascular corrosion casting/scanning electron microscope technique was used to investigate differential uterine elongation at the time of implantation in the rat. Casts were made on Days 4, 5 and 6 of pregnancy (Day 1 = the day of finding spermatozoa in the vaginal smear), implantation sites being identified by an i.v. injection of Evans' blue before casting. By Day 6 the endometrial blood vessel network within each implantation site was widely spaced compared with that in other regions. We suggest that blastocyst spacing is achieved, in part, by differential uterine elongation at the implantation sites, apparently as a result of tissue hydration subsequent to increased vascular permeability.