Rat livers perfused with 0.5% Triton X-100 for 15 to 90 min followed by 1% sodium dodecyl sulfate for 30 min were studied by electron microscopy and polyaerylamide gel electrophoresis. After 15 min of perfusion, a rich network of intermediate filaments and microtubules was visualized in the cytoplasm of hepatocytes. Using stereopairs, branching was visualized. Connections of filaments were noted with nuclei, centrioles, microtubules, vesicles, and rough endoplasmic reticulum. The existence of connections supported the concept that intermediate filaments may function to integrate mechanically the cytoplasmic space as postulated by Lazarides.