Age-related or senile human nuclear cataracts were examined using electron microscopy of thin sections prepared from thick vibrating-knife microtome sections of nuclei extracted by extracapsular surgery. The use of extended aldehyde-tannic acid fixation of 80-120-microns thick vibrating-knife microtome sections overcame the difficult problem of preserving the hardened nuclear core of aged lenses. Comparisons were made between a typical nuclear cataract, containing a central opacity and a transparent rim, and a more advanced, or mature, completely opaque nuclear cataract. The typical nuclear cataract contained no obvious cell disruption, cellular debris, or objects that readily could explain the central opacity. The fiber cells had intact uniformly stained cytoplasms with well-defined plasma membrane borders and gap junctions. The transparent rim and the nuclear core appeared similar, except that fiber cells in the nucleus were more condensed with more elaborate intercellular interdigitations. The mature cataract showed various types of cell disruption in the perimeter but not in the core of the nucleus. These disruptions were globules, vacuoles, multilamellar membranes, and clusters of highly undulating membranes. Because these potential scattering centers were not found in the nuclear core, they probably were not the sole cause of the observed opacity. Other potential scattering centers found throughout the mature cataract nucleus included variations in staining density between adjacent cells, enlarged extracellular spaces between undulating membrane pairs, and protein-like deposits in the extracellular space. Similar features, although less pronounced, were present in the typical nuclear cataract. It was concluded that massive cell disruption is not essential to the formation of a central nuclear opacity. Subtle structural changes, especially small fluctuations in protein density between adjacent cells and alterations of the membranes and the extracellular space, probably contribute significantly to the central opacities in human nuclear cataracts.