Adult and pubertal female mink mated on 4 March were administered 2.5 mg medroxyprogesterone acetate (MPA) or control vehicle on 9-11 March. Pregnancies were shorter (P less than 0.01) in adult (60 +/- 2 days) than in pubertal (72 +/- 0.7 days) control animals. Treatment with MPA reduced (P less than 0.01) the length of pregnancy in both adult (51 +/- 1 days) and pubertal (60 +/- 2 days) mink but did not significantly affect serum progesterone levels on 3-4 April. Observations at laparotomy during April confirmed the effects of age and treatment on the time of implantation and suggested that implantation sites become grossly visible 28 days before parturition. It is concluded that administration of a progestagen during early embryonic diapause can advance the time of implantation in mink.