The progesterone concentration in the peripheral plasma of ewes throughout pregnancy has been determined by a protein-binding method.Plasma progesterone concentrations during the first 50 days of pregnancy (2-3 ng./ml.) were not significantly higher than peak concentrations during the luteal phase in cycling non-pregnant ewes, but there was no decrease in the concentration 15-20 days after mating as occurs in non-pregnant ewes.Between 50 and 120 days after mating the plasma progesterone concentration increased steadily to values 2-5 times that found in early pregnancy. These high concentrations were maintained until lambing. A decrease in progesterone concentration during the week preceding lambing was usually, but not always, observed.Mean plasma progesterone concentrations during the last 50 days of pregnancy in ewes with twins were approximately twice those in ewes with a single foetus.