Xenobiotics have the potential of disrupting the differentiated function of the mammary gland, that is, milk secretion, by inhibiting mammary development, by directly interfering with the secretory function of the mammary alveolar cell, by altering the hormonal milieu that supports lactation, or by impeding nutrient transport to the mammary cell. Although solid information is lacking in most of these areas, available research suggests that antiestrogens may interfere with mammary development, that oxidants and phorbol esters may inhibit milk secretion, that ergot alkaloids and alcohol may alter the hormonal milieu, and that dioxins may interfere with lipid transport to the mammary gland. Models for the investigation of these and other environmental toxins and drugs are suggested.