Groups of ovariectomized rats were taken as controls or given hormonal treatment mimicking the successive steps in the sequence of ovarian secretions leading to implantation. Total endometrium or separated epithelium and stroma were incubated in vitro with [35S]methionine. Dissolved proteins were submitted to two-dimensional polyacrylamide gel electrophoresis (pH range 5-7), followed by autoradiography. Priming with oestradiol (2 days) and subsequent treatment with progesterone (3 days) enhanced the synthesis of 12 and 14 polypeptides, respectively, which are specific for each of these treatments. Progesterone also suppressed the production of 10 oestrogen-dependent proteins both in the epithelium and the stroma. When an oestrogen-progesterone-oestrogen treatment was given, synthesis of all but 4 of the progesterone-induced polypeptides in the epithelium was inhibited while 5 of the proteins abolished by progesterone in this tissue compartment reappeared. These results are compatible with a mechanism of implantation acting at the epithelial level by lifting of intrauterine inhibition and stimulation by embryotrophic substances.