Multiple ejaculatory series may be critically important for successful reproduction by multiparous old females near the end of their active reproductive lives. Multiple series were essential for successful pregnancy, apparently because more than 1 series was necessary to induce functional luteal activity. Females receiving just 1 series failed to show the cessation of regular estrus cycles which characteristically follows induction of a functional luteal phase. Direct measurement of plasma progestins by radioimmunoassay revealed that only those females receiving more than 1 series had circulating levels significantly higher than unmated, normally cycling controls. Administration of exogenous progesterone increased significantly the incidence of successful pregnancy among females allowed to mate through only 1 series. In addition to facilitating the occurrence of pregnancy, multiple series also increased both the number of eggs released at ovulation and the number of embryos present and developing 12 days later. Multiple series apparently may function as a behavioral mehanism for prolonging the female''s active reproductive life.