On the control of prostaglandin release during the bovine estrous cycle. Effects of progesterone implants.

Progesterone implants were inserted subcutaneously on day 12 of the estrous cycle in three heifers. The implants were known to release an amount of progesterone sufficient to maintain the peripheral blood plasma at a level of about 1 ng/lm. All three heifers showed luteolysis in a similar way with peaks of 15-keto-13, 14-dihydro-PGF2 alpha of normal magnitude and duration. Concomitantly, progesterone levels declined, but due to the presence of the implants, progesterone was maintained at a level of about 1 ng/ml. The release of prostaglandins, however, continued throughout the experimental period and did not cease until the implants were removed.