Two experiments were conducted to determine whether administration of progesterone during early pregnancy affects fetal growth in sheep and if any effect is specific to the days of treatment. In the first experiment, Merino ewes were randomly allocated to four treatment groups and inseminated at a synchronized oestrus. Three groups received progesterone on days 1–3, 3–6 or 1–6 of pregnancy while the fourth group was untreated. Concentrations of progesterone in peripheral plasma increased ( P < 0.05) in all treatment groups. Fetal growth (to day 74) was greater in all treatment groups than in the control group ( P < 0.001) and was greatest when treatments started on day 1. Pregnancy rate was not affected by progesterone treatment on days 3–6, but was reduced ( P < 0.05) when treatment began on day 1. In the second experiment, embryos that had been exposed to either a normal (control) or a high concentration of progesterone on days 1–3 were randomly transferred, within groups, to recipient ewes that had or had not been treated with progesterone on days 1–3. In another group, embryos were exposed to a high concentration of progesterone on days 1–3 and the oviducts of the ewe were ligated. An increase in fetal mass was observed in the recipient group that had been treated with progesterone ( P < 0.01) but was not observed in the initial group treated with progesterone. A greater fetal mass was also obtained when embryos that had been ligated in the oviducts of ewes treated with progesterone ( P < 0.05) were transferred. This effect occurred irrespective of whether the final recipients received progesterone. These findings indicate that progesterone supplementation to ewes during the first 3 days of pregnancy enhances the growth of surviving fetuses. It is concluded that progesterone treatment or ligation of the oviducts during the first three days of pregnancy induce changes in embryo development that affect subsequent development.