Studies on serum oestrogen and progesterone levels during the oestrous cycle and early pregnancy in mares.

Concentrations of progesterone and oestrogens were determined by radioimmunoassay in the peripheral blood of 22 Percheron and Breton breed mares from the 6th day of oestrus to the 150th day of pregnancy. Periodical variation patterns for the mean values of oestrone, oestradiol 17beta and total oestrogens in the cycling mares were found, with two peaks on the third day before and the 15th day after ovulation, and one depression on the 6th day of oestrus. In pregnant mares, the concentrations of oestrone and oestradiol 17beta increased rapidly (P less than 0.05) after Day 105 of gestation. Progesterone levels declined after Day 18 of the cycle in cycling mares, whereas they increased in the pregnant mares. Marked changes in the ratio of total oestrogen to progesterone concentrations were found in cycling mares, especially at oestrus, but in pregnant mares the ratio always remained below 1.00.