The microfilaments of hepatocyte are distributed throughout the vicinity of cell membranes, especially numerous around the region of bile canaliculus, and provide the maintenance of cell shape, cellular wall tension, canalicular motility, the secretion for bile, etc. To evaluate the relationship between the microfilament and alteration of cell shape, we examined the morphological changes of cultured rat hepatocytes, following treatments with phalloidin or cytochalasin D with fluorescent and electron microscopes. 1. In the fluorescent micrographs, actin microfilament was distributed near the plasma membrane and bile canaliculus. 2. Both drugs, phalloidin or cytochalasin D, produce the cytoplasmic protrusions from the surface. Their shapes were pedunculated with narrow neck or bulged with broad base, respectively. 3. In the phalloidin treated group, cytoplasmic protrusion was seperated from the internal cytoplasm by microfila-ments networks at the narrow base. In contrast, in the cytochalasin D treated group, cytoplasm was bulged with broad base and kept in direct continuity with the canalicular ectoplasm. 4. Pericanalicular ectoplasm of phalloidin treated group was widened and accumulated with microfilaments. But, bile canaliculus of cytochalasin D treated group was markedly dilated and devoid of microvilli, and the ectoplasm was almost disappeared. Considering above results, dysfunction of microfilaments leads to the structural changes and inhibition of bile secretion of hepatocytes.