The nonspecific liver esterases of some representatives of vertebrate classes have been analysed by vertical starch gel electrophoresis. The enzyme showed amolecular heterogeneity and a species-specific resolution pattern. Some sex-related differences in the relative activity of some isozymes, moreover, have been observed in the species examined. In Rana esculenta, Lacerta s. sicula and Gallus domesticus, experiments of gonadectomy and/or hormonal therapy with sex hormones showed that the sex-related pattern could be under gonadal endocrine control.