Facilitatory and inhibitory effects of progesterone on sexual responses of spayed guinea pigs.

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Abstract
Onset of estrous behavior in estrogen-primed female guinea pigs was facilitated by progesterone. For an extended period following termination of receptivity, Ss were refractory to stimulation by progesterone alone or to sequential treatment with estrogen and progesterone. Facilitatory effects of progesterone occurred only following a conditioning period with estrogen; pro?gesterone given prior to the completion of this conditioning process produced only inhibitory effects. Experimental evidence is presented suggesting that some inhibitory effects of progesterone depend on an interaction of progesterone with estrogen. Results are discussed in terms of the hypothesis that progesterone may act as a competitive inhibitor of estrogen at neural sites.