The distribution of myoepithelium and smooth muscle in the lactating mammary gland of the goat has been examined by a new technique of silver impregnation which is applicable to sections up to $100\mu $ in thickness. Myoepithelium covers the stromal surface of the epithelium of the alveoli, ducts and cisterns of the entire gland, and is thus much more abundant than is generally realized. Smooth muscle forms scattered inter-lobular bundles closely associated with the blood vessels. The theory that myoepithelial contraction is the principal factor concerned with 'let-down' and the ejection of milk is examined; other factors such as inter-lobular smooth muscle contraction, vascular changes, and elastic recoil of the stroma appear to play minor roles, if any, in this phenomenon. Hitherto, it has been assumed that myoepithelial cells are contractile because they bear structural resemblances to smooth muscle fibres. With the new technique structural changes have been found in the myoepithelium of contracted as compared with distended alveoli and ducts. These changes, together with the general orientation of myoepithelial cells, and the precise relationship between these cells and the folds in the secretory epithelium from contracted glands, are consistent with the assumption that myoepithelium is the contractile tissue in the mamma which responds to a neurohormonal mechanism involving oxytocin.