Alteration of serum estrogen levels in pregnant mares

The estrogen levels in the peripheral blood serum of 5 mares were estimated by readioimmunoassay throughout their gestation period. Blood samples were collected from jugular veins, and the radioimmunoassay method, originated by Makino, had sufficient accuracy and reliability for the estimation. The result obtained was summarized as follows. The serum concentration of E, remained below 40 pg/ml from Day 0 to Day 75, then increased to 190.5 pg/ml on Day 120, and sharply up to the average value of 3682.2 pg/ml on Day 210, and the high levels continued for about a month. It then declined sharply towards 107.9 pg/ml on Day 300, with the fluctuation around Day 255 to 270. The E2 concentration showed the similar pattern to that of E1: it remained below 15.2 pg/ml from Day 0 to Day 75, 46.5 pg/ml on Day 120, then began to increase rather sharply up to Day 210, and the average highest value obtained was 1740.6 pg/ml. From around Day 210, the average E2 concentration remained high for about 2 months, then it declined sharply to 71.3 pg/ml on Day 285. The E3 level showed large variation within and among individuals, and no correlation was observed throughout the gestation period. (Kobatake, H.)