Mammary gland pieces from lactating mice were incubated for various times up to 4 h in collagenase and then additionally for up to 1.5 h in a mixture of collagenase and hyaluronidase. Tissue then was fixed and prepared for scanning electron microscope viewing. Monitoring of time of enzymic hydrolysis allowed visualization of connective tissue sheets, secretory lobules, blood vessels, alveoli encased in and devoid of basement membrane, secretory epithelia, alveolar milk ducts, and myoepithelial cells. Scanning electron microscope viewing of alveoli in cross section showed good internal micro-anatomy including a valve-like arrangement of secretory epithelia at the point of milk duct attachment. Morphology of myoepithelial cells and their relationship to secretory cells and to each other was presented. There was no evidence that myoepithelial cells were interconnected continuously throughout the tissue, but they interlaced over an alveolus; thus, the concept of a limited physiological syncytium was advanced.