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Publisher Summary The apical plasma membrane of secretory cells undergoes considerable transformations during release of the products of secretion. Secretory vesicles derived from Golgi cisternae contact and fuse with the apical plasma membrane with formation of a small bilayer diaphragm, which separates the vacuolar content from the acinar lumen of the gland. Rupture of this transient diaphragm results in the release of the secretory products and incorporation of the secretory vesicle membrane in the apical segment of the plasma membrane. This process has been investigated in different types of cells. In the lactating cells of the mammary alveolus, the spectrum of transformations observed at the apical surface is wider. In addition to the release of proteins and other secretory products by exocytosis, other events occur at the apical membrane during the secretion of fat droplets. Milk fat droplets are released into the alveolar lumen surrounded by a membrane envelope derived from the cell membrane. The process results in the exportation of plasma membrane, which can be easily isolated from milk. This membrane system represents a useful model to study the dynamics of the apical surface of a secretory cell, as well as other membrane events of wider biological significance. The observation of lactating tissues with freeze fracture techniques provides imaging of the process and allows the analysis of the structural rearrangements of these membranes.