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Progesterone injected into pregnant pigs from the time of ovulation until Days 10 to 13 of gestation does not prevent formation of corpora lutea. Injections of progesterone beyond Days 12 to 16 of pregnancy, i.e. after corpus luteum formation has been completed, results in complete and rapid destruction of the formed corpora lutea.In contrast, progesterone injected into rats throughout the entire pregnancy or pseudopregnancy has no visible or measurable effect on corpora lutea.Since in both rats and pigs a luteotrophic hormone (lth) is presumed to be involved in the formation and maintenance of corpora lutea, it appears that progesterone is able to inhibit lth release and hence maintenance of corpora lutea during pregnancy, but not during the luteal phase of the cycle. The possible theoretical implications of these findings concerning the role of lth in maintenance of corpora lutea and the mechanisms of release of lth in rats and in pigs are discussed.

High doses of progesterone can apparently inhibit secretion or release of hypophyseal hormones, including the luteotrophic hor¬ mone (lth). Injection of progesterone into non-pregnant and pregnant guinea-pigs led to partial destruction of formed corpora lutea but did not prevent their initial formation.The data presented suggest that in the guinea-pig lth is released about the time of ovulation, but for a period of less than 3 days. This temporary release of lth is sufficient to initiate formation and function of corpora lutea. No further lth support seems to occur in non-pregnant females.In pregnant animals, a neurohumoral signalling mechanism initiated by intrauterine events of implantation is assumed to call forth a second release of hypophyseal lth, which is responsible for the maintenance of corpora lutea during the remainder of their functional life span during gestation. Prolactin is not luteotrophic in guinea-pigs.

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