Mouse hepatic cells appear in thin sections as polygons with six or more sides. The plasma membrane covering these sides may contact either bile canaliculi, the narrow intercellular space, the space of Disse or extensions of the space of Disse between adjacent cells. The plasma membrane covering microvilli in bile canaliculi and the space of Disse is thicker than that in contact with the narrow intercellular space. Bile canaliculi, which contact about 6% of the perimeter of each cell, are each separated by tight junctions from the narrow intercellular space. This space contacts more than one-half the perimeter of each cell and is about 220 Å in width. It is continuous around the occasional studlike junctions which occur, but is interrupted at frequent intervals by circumscribed tight junctions, and occasionally by desmosomes. The narrow intercellular space is in free communication through the space of Disse with the plasma space. An interstitial fluid space, separate from the plasma space, does not occur in the liver lobule. Protein molecules from plasma enter hepatic cells in both coated and pinocytotic vesicles. These vesicles are derived from invaginations of the plasma membrane that borders the narrow intercellular space and the spaces between microvilli in the space of Disse. Pinocytotic vesicles may also incorporate fat droplets into hepatic cells.