Cervical mucus is a glycoprotein gel whose biological functions depend upon its macromolecular architecture. Using freeze-substitution fixation techniques, we have used transmission electron microscopy to examine the fine structural aspects of mucus, before and after unidirectional physical shearing, and during its interaction with sperm. The microstructure of mucus that has not been directionally stretched consists of a homogenous pattern of interconnecting electron-dense elements. The thickness of the primary structural elements varies from 0.04 to 0.5 microns, giving the impression that the elements have fibrillar but also ribbon-like properties. This dimension is smaller than the sizes depicted by prior studies using scanning electron microscopy. Within the mucus interior, the interstitial distance between adjacent primary elements ranges from 0.5 to 0.8 microns, and is filled with a fibrous network of secondary structural elements. This interstitial dimension is also significantly smaller than that suggested by prior electron microscopic work. At the exterior borders of the mucus, the interstitial sizes are reduced. After physical stretching by forceps, the mucus microstructure undergoes a radical deformation. In some specimens, the primary structural elements become longitudinally aligned and are less interconnected, with compaction at the exterior mucus borders. In other specimens, the primary structural elements become laterally compressed along the longitudinal axis; within the regions of compression, the intrapore diameter is reduced to less than 0.1 microns. These compressed regions will often exclude spermatozoa. Individual sperm deform the local mucus microstructure. The mucus directly anterior to the sperm head is stretched, and the mucus adjacent to the bending flagellum is compressed on the forward side and stretched on the opposite side.