The uterine uptake of tritiated hormones, oestradiol and progesterone, was studied at the time of ovum implantation in the rat. Macromolecular dyes such as trypan blue were used to distinguish implanted sites from the unimplanted ones. A differential uptake was found between the implanted and unimplanted sites in normal 6th day pregnant females. Oestradiol was retained more at the unimplanted sites, whereas its half-life was approximately the same at the two levels. On the other hand progesterone was found in greater amounts in the implanted sites due to a difference in half-life for each site (30 min for unimplanted sites, 40 min for implanted ones during the first 30 min following injection). When endogenous oestradiol retention was suppressed by ovariectomy, higher uptake of [3H] oestradiol in unimplanted sites was not found. These results suggest a heterogeneity in the binding sites of oestradiol in the pregnant uterus.