Morphometric evaluation of liver in the Japanese medaka (Oryzias latipes) was done to define components quantitatively. Analysis was performed in individual fish at: day of hatch (larval), 3-6, and 6-9 months, and 1 year. From analyses of hematoxylin and eosin-stained paraffin sections, the volume densities of hepatocytes, blood vessels (inclusive of sinusoids), intrahepatic bile ducts and ductules, and other components (connective tissue, melanomacrophage centers) were determined. No significant age-related changes were seen. To determine those structural parameters which undergo change 66 and 116 days following a 10-day exposure to methylazoxymethanol acetate (0.5 ppm in water), we performed analyses as above on livers of treated and paired control fish. The stained paraffin sections demonstrated a significant decrease in the volume densities of hepatocytes and blood space and an increase in volume density of bile ducts 66 days after exposure. Although these whole liver values returned toward normal 116 days postexposure, significant alterations in hepatocytes were still seen when toluidine blue-stained semithin sections of Epon-embedded liver were studied. The nuclear-to-cytoplasmic ratio was increased as was stainable material at the cell periphery. The most dramatic change was in the number of hepatocytes per cubic centimeter of liver which was increased by 300% over controls in animals 116 days postexposure. Morphometric data were compared with subjective evaluation at the light and electron microscopic levels.