Mice of the CHI and Brown Belt stocks received subcutaneous pellets of 17α-hydroxyprogesterone or progesterone and then were ovariectomized at various stages of pregnancy. The average daily absorption of progesterone from the pellets which was required to ensure fetal survival to term diminished as pregnancy progressed. 17α-Hydroxyprogesterone pellets were ineffective in maintaining pregnancy in castrate mice. When the day of finding the vaginal plug was counted as day 1, duration of pregnancy in the intact untreated CHI mouse was usually 19 or 20 days. In the Brown Belt mouse, it was usually 20 or 21 days.