Estrogen and progesterone administered in sequence are more effective than estrogen alone in inducing sexual receptivity in the spayed [female] mouse. The optimum amt. of progesterone is 0.05 I.U., and the optimum time interval between injs. is 48 hrs. The mating behavior induced by the synergistic action of progesterone and estrogen appears to be more normal in character than that induced by estrogen alone. It is concluded, therefore, that the mouse is another of the sp. in which estrus is the result of a conditioning effect of estrogen followed by the action of progesterone secreted by the ovarian follicle prior to ovulation.