In the process of hepatocarcinogenesis induced by diethylnitrosamine (DENA) in rats, the hepatocellular ultrastructure and G-6-Pase reactions in hepatic nodes were observed by electron microscope. The results are as follows: As compared with normal hepatocytes, cell junctions were fewer, even disappeared in some areas and intercellular spaces were wider; in some cells, nuclear membranes invaginated into the nucleoplasms frequently, micleoli were enlarged, mitochondria appeared swollen and their cristae were scanty- and short, and depolymerized ribosomes dropped off the dilated rough endoplasmic reticulums; in some seriously diseased cells, nucleoli were enlarged, abundant free ribosomes were present, but the' other organelles were in lower differencial state. G-6-Pase reactions were positive before the 8th week of DENA induction and negative after the 12th week. These suggest the hepatocellular metabolic disturbance and low differenciation.