Pouch young were removed from lactating tammars to terminate embryonic diapause. Uterine metabolism was assessed at periods afterwards by incubating endometrial explants with [3H]leucine, and measuring the incorporation into acid-soluble material. Blastocysts were incubated with [3H]uridine to assess uptake and incorporation into acid-soluble material. Uterine reactivation, shown by an increase in the rate of leucine incorporation into secreted protein, was evident by Day 4 after removal of pouch young and was significantly more in both secreted and tissue protein by Day 6. Both continued to increase in gravid and non-gravid uteri up to Day 12. By the end of pregnancy (Day 26) uterine metabolism in the gravid uterus produced 2-3 times more secreted protein than in the non-gravid uterus, demonstrating a local feto-placental influence on the uterus. Tissue incorporation had declined in endometrium of gravid and non-gravid uteri by Day 26. Day 5 embryos were metabolically more active than in quiescence, although expansion of the embryos was not seen until Day 9. The early reactivation of the uterus and embryo from diapause suggests that it is not triggered by the previously described peaks of progesterone and oestradiol in plasma at Day 5, although there may be an earlier, increased sensitivity to these steroids which allows uterine reactivation to precede changes in peripheral plasma concentration.