and

The effect of exogenous progesterone or megestrol acetate (MGA) on cervical extensibility and on the concentrations of prostaglandin (PC) F in utero-ovarian plasma and progesterone, estrone sulphate and unconjugated estradiol-1 713 concentrations in jugular plasma was studied over Days 238-242 in pregnant cows given intrafetal infusions of dexamethasone to induce premature delivery. All 1 1 cows used in the study were given continuous intrafetal infusions of dexamethasone at a rate of 3.3 mg/day to induce premature labor. Progesterone was infused continuously at a rate of 400 mg/day for 10 days into the jugular vein of 3 of the 11 cows while another 3 were given MGA at a rate of 20 mg/day. In the cows given dexamethasone alone premature parturition was induced in 2.5-4.5 days and all calves were born alive and well. However, in the progestogen-treated cows, dexamethasone failed to induce premature delivery. The cervix of these cows remained firm and unyielding during the time of the progesterone infusion. Following the withdrawal of exogenous progestogens the cervix became soft and compliant and the cows delivered within 0.5-1.5 days, but only 2 of the 6 calves were alive at birth. Hormone changes in the cows given intrafetal infusions of dexamethasone included a marked rise in utero-ovarian PGF and a concomitant decline in jugular progesterone concentrations. Plasm