Summary. Three tests are described, two each with thirty-six entire and one with fifty-six spayed ewes, in which the action of clomiphene citrate was characterized in the sheep. It was found to suppress pituitary activity, to induce behavioural oestrus and to have a grossly stimulatory action on the uterus in that it induced a high incidence of hydrops uteri. It is concluded that clomiphene acts in the ewe as a weak oestrogen but differs qualitatively from true oestrogen in its effect on the uterus.