Eight ovariectomized pony mares were used to test the effect of various doses of progesterone (0, 50, 150, 450 mg/day, in oil, i.m., for 10 days) on progesterone and LH in the peripheral circulation, and on total protein and uteroferrin in uterine secretions. Progesterone increased uteroferrin, but there were no differences amongst doses of progesterone. Progesterone treatment decreased LH, and tended to increase total protein. Eighteen ovariectomized mares were given vehicle, oestradiol (10 mg/day, in oil, i.m.), progesterone or progesterone + oestradiol for 28 days. Both the last two steroid treatments significantly increased total protein and uteroferrin in the uterine secretions, compared to vehicle or oestradiol alone. Progesterone + oestradiol increased uteroferrin, but not total protein compared to progesterone. Nine ovariectomized progesterone-primed mares were used to compare systemic and intraluminal administration of oestradiol. There were no differences between routes of administration of oestradiol. In conclusion, administration of progesterone increased total protein and uteroferrin in uterine secretions, and progesterone + oestradiol increased them further.