This study was designed to investigate the ultrastructural alterations of the hepatocyte and bile canaliculus of the fasted mice with transmission and scanning electron microscopes. The morphometry was also carried out for the caliber of the bile canaliculus and the number, length and thickness of the microvillus. The hepatocyte observed in the three day fasting group showed ultrastructural images of active function. The dilated bile canaliculi, especially of type II were increased in number as compared with those seen in the normal group. However, the hepatocyte observed in the six day fasting group showed ultrastructural images of inactive function. The bile canaliculi without dilation (type I) were increased in number. The number of microvilli were identical with one another among the different types of bile canaliculi, while their length and thickness were reduced in the dilated bile canaliculi. From the evidence, the luminal size of the bile canaliculi seems to be easily changeable according to the functional state of the hepatocyte. However, the microvilli may not be changed in number but may be changed length and thickness when the bile canaliculi are dilated.