1. The action of ephedrine on the smooth muscle of various uteri is described.
2. Ephedrine always contracts uterine smooth muscle and the augmentor effect is abolished by ergotamine.
3. The inhibitory effect of adrenaline on uterus is converted into an augmentor effect by exposure of the uterus to ephedrine.
4. The response of the blood pressure of chloralosed cats to adrenaline is diminished by administration of ephedrine.
5. Some theoretical implications are discussed.