The permeability of mammary gland capillaries to ferritin was investigated by transmission electron microscopy. In virgin mice, the concentration of tracers in perivascular spaces increased with the advance of time after injection. The ferritin never passed through the intercellular junction of the endothelial cells. In the early to middle stages of lactation, numerous ferritin particles were observed in the basal cytoplasm of alveolar epithelial cells after only 1 min of circulation. These findings indicate that a large amount of materials quickly pass through the capillary walls during these periods for milk production. The number of vesicles labelled with ferritin per μm² of endothelial cytoplasm decreased remarkably in the late stage of lactation. This result suggests that the permeability of vesicles declines along with the degeneration of alveolar cells.