Introduction: The morphogenesis of the glomerular nephrosis is not yet clear. For example, it is not established whether different noxious agents cause different cellular reaction patterns and how the podocytes with their trabeculae behave. Therefore we studied kidney glomeruli during the Daunomycin-nephrosis, a new model of the nephrotic syndrome, with the transmission (TEM) and scanning electron microscope (SEM). Materials and Method: Female Wistar rats (150–180 g) received a single intravenous injection of 15 mg/kg body weight Daunomycin in 1.0 ml phys. NaCl. After 6 and 24 hours and 7, 14, 29, 36, and 50 days the rats were perfused in vivo via the abdominal aorta under ether anesthesia. The perfusion solution was 3 % glutaraldehyde + 3 % saccharose +3% dextran (15–20,000 m.w.) in phosphate buffer (0.1 m; pH 7.2), the perfusion pressure was 100–120 mm Hg. The 5 min fixed kidneys were removed, cut into small pieces and 2–3 mm thick slices, and were incubated for another two hours within the same fixative. Postfixation with OsO 4 was performed after several washings with the buffer. For TEM the small pieces were embedded in Epon 812. The ultrathin sections were stained with uranyl acetate and lead citrate. For SEM the thick tissue slices were dehydrated through a graded alcohol series followed by a vacuum desiccation. To prevent surface charging the specimens were coated with a layer of 100 Å carbon and 300 A gold. They were studied with the JSM-U 3 (Jeol) and with the Stereoscan (Cambridge). The proteinuria (see table) was measured with the biuret test after Weichselbaum using metabolism cages for the rats. Results and Discussion: Although the proteinuria begins about the 7th day, morphological alterations can be observed as soon as 6 hours after Daunomycin-injection: several endothelial and mesangial cells, seldom a podocyte, reveal homogeneous chromatin structures of their nuclei and a loss of the nucleoli. After 24 hours nearly all of the cells of the endothelium and mesangium as well as some podocytes show these changes (fig. 1), which persist until the 7th day. 14 days after Daunomycin-injection the nuclei possess normal chromatin structures (fig. 4) and often a great nucleolus. At the end of our study the endothelium is entirely normal while the mesangium is slightly enlarged and may have incorporated fat and protein droplets like the podocytes. With the SEM first swellings of the podocyte trabeculae are observed 6 hours after Daunomycin (fig. 9). These are more pronounced after 24 hours (fig. 10) and at the 14th day most of the podocytes possess only a few short and thick trabeculae (fig. 12), but some pedicels are always preserved (fig. 13). Most podocytes are rounded (fig. 12), but some are flattened and spread out (fig. 13). While after 50 days—in spite of the still high proteinuria—many podocytes look like normal ones in the TEM, the SEM shows a more irregular pattern of their trabeculae and pedicels compared with normal rats (fig. 15, comp. with fig. 8, 11). Whether this irregular pattern of the podocytes has functional consequences is not known. First inclusion bodies and vacuoles are seen with the TEM at the 7th day (fig. 3). They may give rise to mulberry forms (fig. 15) or protrusions similar to those of the nuclei (fig. 14) in the SEM. The SEM shows very clearly that normal and pathological cell forms—as well as pathological findings of different severity (fig. 15)-may exist in immediate neighbourhood (comp. fig. 10 with fig. 11). A particular finding are bundels of filaments (fig. 4–6) in the podocytes situated near the basement membrane or surrounding vacuoles. They often contain rhythmic densities (fig. 5) or may be cross-striated (fig. 6). We suggest that these filaments demonstrate the activation of a contractile system within the podocytes.