The numbers of granulated metrial gland cells, the percentage of them incorporating tritiated thymidine and the numbers of granulated metrial gland cell precursors in a defined area of the metrial gland, and the cross sectional area of the metrial gland, have been determined in mice ovariectomised on Day 8 of pregnancy. The effects, on these parameters and on fetal survival, of progesterone treatment, given at two levels (0.5 mg or 1 mg twice daily) and started immediately after ovariectomy or after 6, 10 or 24 hours delay, have been studied. Granulated metrial gland cell number, DNA synthetic activity and metrial gland size decreased rapidly after ovariectomy but there was a significant increase in the number of granulated metrial gland cell precursors in the first 24 hours. Progesterone treatment, at the higher dose level started immediately after ovariectomy, resulted in significantly more granulated metrial gland cells in the area analysed two days after ovariectomy although a normal proportion of them were in DNA synthesis. Immediate progesterone treatment, at the higher dose level, was able to prolong the DNA synthetic activity of granulated metrial gland cells until Day 19 and this probably accounted for the significantly greater numbers of granulated metrial gland cells in these mice than in control mice, at this time. Delaying progesterone treatment at the higher level by 24 hours significantly reduced the size of the metrial glands at Day 10 but there was no effect on the cellular composition of the area analysed. Delaying the lower dose of progesterone treatment by 24 hours also resulted in small metrial glands and was associated with significantly fewer granulated metrial gland cells in the area analysed but significantly more of them were synthesising DNA. The observations are discussed and an attempt is made to relate the effects on individual fetal and placental survival to the effects on the corresponding metrial glands.