The transforming growth factors beta (TGF-beta) are multifunctional regulators with diverse effects on a variety of developmental processes and differentiated functions. In the mammary gland, a considerable amount of evidence has accumulated indicating that TGF-beta plays a critical role during several phases of the mammary cycle. TGF-beta regulates growth and patterning of the mammary ductal tree in the virgin mouse. During pregnancy, TGF-beta is required for alveolar development and functional differentiation, while at the same time inhibiting secretion of milk proteins. At parturition this inhibition is lifted, permitting initiation of lactation.