Effect of Progesterone and Oxytocin on the Pituitary-Ovarian Relationship in Heifers

Summary. The effects of oxytocin and of progesterone on the pituitaryovarian relationships were studied in sixteen non-pregnant heifers. Subcutaneous injection of 100 mg of progesterone in corn oil daily for 35 days from Day 7 of the oestrous cycle depressed both the size of the largest follicle and the total weight of follicular fluid. The fsh level was significantly raised after progesterone injections as compared to Day 1 of the cycle but not as compared to Day 7. The progesterone treatment had no significant effect on lh level. It was concluded that progesterone curtails the production, and consequently the release, of fsh and lh. The injection of 150 U.S.P. units of oxytocin daily from the day of oestrus to Day 6 of the cycle produced a smaller corpus luteum (2·51 versus 4·40 g, P < 0·05) and a reduced concentration of progesterone (14·6 versus 29·0 μg/g, P<0·05). The oxytocin treatment had no significant effect on the hypophyseal levels of fsh and lh. It did not affect the size of the largest follicle or the total follicular fluid weight. The corpus luteum of the previous cycle, still present on the day of oestrus, was small (1·74 g) but contained a detectable amount of progesterone (2·8 μg/g.