Changes in ultrastructural distribution patterns of nonspecific esterases (E.C. 3.1.19) are described quantitatively by means of morphometry. Esterases were demonstrated with O-acetyl-8-hydroxyquinoline (Q-O-2) and S-acetyl-8-mercaptoquinoline in livers of normally and exclusively fructose-fed mice. Conditions are discussed, under which the quantification of the ultrastructural products of enzyme histochemical reactions may be possible. Smooth endoplasmic reticulum and rough endoplasmic reticulum exhibit no alteration in enzyme distribution with both substrates since the enzyme-occupied proportion of each compartment remains the same despite an overall decrease of both compartments. Likewise an increase of O-acetyl-8-hydroxyquinoline-esterase at fat droplets corresponds to the increase in total surface of the fat. S-acetyl-8-mercaptoquinoline-positive fat surface however reveals an increase far beyond that of the total fat surface. The results support the hypothesis that a variety of esterases with different substrate spectra are present at the subcellular level in different cell compartments.