Uterine secretions were analysed after labelling with L-14,5-3H]leucine. During pro-oestrus major protein peaks were seen with approximate mol. wt 2.0 X 10(4), 2.7 X 10(4), 4.0 X 10(4), 5.7 X 10(4), 7.8 X 10(4) and 13.0 X 10(4). Qualitative changes were observed during the first 4 days of pregnancy. On day 4 p.c. secretions revealed two prominent peaks with mol. wt 6.7 X 10(4) and 12.5 X 10(4), four peaks between 2.0 and 4.7 X 10(4) and one at 8.5 X 10(4) and one at 20.0 X 10(4). Profiles similar to those on mimic Day 4 p.c., and implantation, were induced in ovariectomized females by administration of exogenous progesterone and oestradiol, whereas the proteins detected after administration of exogenous oestradiol alone were essentially similar to those found at pro-oestrus. Ovariectomized mice treated with the steroid sequence to mimic Day 4 p.c. profiles but not given the final injection of oestradiol showed no detectable labelled proteins. Injection of progesterone or oestradiol alone, in the final sequence of injections given to ovariectomized pregnant females treated with progesterone, did not induce blastocyst implantation. These studies indicate a synergistic relationship between progesterone and oestrogen in the secretion of uterine macromolecules.