Plasma Levels of Progestin During Absorption of Progesterone Pellets in Mice †

Prolonged, continuous absorption of a steroid hormone can be readily accomplished by implantation of the hormone in the form of a compressed pellet or tablet. This study was undertaken in order to determine what plasma levels of progestin result when progesterone is absorbed continuously over long periods. PROCEDURE Pure progesterone, without excipient, was compressed into cylindrical pellets according to a standard procedure whereby each pellet has the same diameter and each is subjected to the same amount of compression.2 The pellets were roughly 1 mm. in diameter, 6 mm. long, and 10 mgm. in weight. Some pellets were divided by cutting across the long axis. Prior to implantation each pellet was cleaned with a fine brush to remove any loose particles and weighed. The pellet was then placed in the lumen of a $13 hypodermic needle, the needle was pushed at least 1 cm. under the dorsal skin of the recipient, and the pellet was discharged with a stylet. The recipients, lightly anesthetized with ether at the time of implantation, were 90 mice of the inbred CHI, BC, A, and CBA strains. The mice had been ovariectomized 16 days before implanta-tion of the pellets and were about 8 weeks old at the time of ovariectomy. Nine of the 90 mice died between implantation and autopsy or for other reasons could not be used.