Continuous milk production is a consequence of a complex interplay of lactogenic hormones and it depends on the suckling stimulus during lactation. Involution is associated with a massive engorgement of the gland with milk followed by apoptosis of secretory epithelial cells and a restructing of the gland. Sealing of a single gland during lactation is sufficient to induce an initial engorgement and a subsequent collapse of alveolar structures and massive epithelial cell death while the other glands of the same animal remain morphologically and functionally in a lactating state. Many markers of involution such as sulfated glycoprotein-2, protein kinase A, transcription factor AP-1 and most notably stromelysin are induced in sealed glands. These findings suggest a cell death pathway which is independent of the systemic levels of lactogenic hormones but which is triggered by an accumulation of apoptosis-inducing factors in the milk, in the lobulo-alveolar structures or by a physical distortion of secretory epithelial cells generated by the engorgement.