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RELEASE OF PROGESTERONE FROM SILICONE RUBBER IMPLANTS IN VITRO, AND THE EFFECTS OF THE IMPLANTS ON PLASMA PROGESTERONE LEVELS IN SHEEP

Ministry of Agriculture, Fisheries and Food, Central Veterinary Laboratory, New Haw, Weybridge, Surrey KT15 3NB

(Received 26th November 1974)

The effects of silicone rubber progesterone implants on plasma progesterone levels in anoestrous ewes have been reported by Symons, Cunningham, Saba & Millar (1974). Plasma progesterone rose from a mean preimplantation value of 0·18 ng/ml to 0·58 ng/ml 24 hr after implantation. The mean level then declined and by 10 days was not significantly different from the preimplantation value. Such short-lived elevations of plasma progesterone would probably not be effective in suppressing ovulation in cyclic sheep. For the synchronization of oestrus in cyclic ewes by inhibition of ovulation, or the induction of oestrus in anoestrous ewes, it would be desirable to maintain plasma progesterone levels above 1 ng/ml for 10 to 14 days. Thus, we have studied the release of progesterone from various silicone rubber implants in vitro, and have investigated