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LH and prolactin were measured in plasma of 5 cows before and after enucleation of the persistent corpus luteum. Two cows returned to normal oestrous cycles following enucleation and did not differ from normal, cycling cows, with respect to hormone levels. Three other ones developed follicular cysts following enucleation and had decreased levels of LH and prolaction as compared with cycling cows. On the day of enucleation the fluctuations in LH-levels were higher than either before or after enucleation.

Trials were conducted with four heifers and twelve dairy cows to study the luteotropic effect of the serum of pregnant mares administered at the rates of 2000 to 3000 units. The effect of serum gonadotropin (PMSG) on ovarian activity was compared with the ovarian activity of control animals and ovarian activity after the administration of hypophyseal hormones. The highest progesterone concentration of PMSG-treated cows ranged from 41.9 to 74.3 ng per ml of milk; in the control group this range was between 32.2 and 43.1 ng per ml and in the group of gonadotropin-treated cows between 31.7 and 44.8 ng per ml. In heifers the progesterone levels increased from 3.56 to 4.58 ng per ml of plasma within 24 hours from the administration of PMSG. After 48 hours from administration the average progesterone concentration increased to 11.02 ng per ml of plasma. The animals which did not respond to PMSG administration by the growth of follicles exhibited an increased secretion of progesterone. The administration of hypophyseal hormones in the luteal stage of sexual cycle did not exert any significant influence on progesterone concentration, but stimulated the growth of follicles.