In this study, we aimed to highlight the influence of anticoagulants on erythrocyte morphometry in different vertebrate species. Anticoagulants are a category of substances that inhibit blood clotting through various mechanisms. Due to this property, they are used to collect blood samples for a wide range of laboratory tests. The literature mentions that the use of anticoagulants produces morphological changes of erythrocytes, thus influencing results. Blood samples were collected from three warm-blooded vertebrate species (horse, rabbit, and chicken) and one lower vertebrate species with nucleated erythrocytes (fish) in vacutainers with Heparin and EDTA (ethylenediaminetetraacetic acid), in a normal concentration and a double concentration. At the time of harvesting, control smears were performed. In order to be able to compare the effects produced by anticoagulants on the morphology of erythrocytes, they were evaluated morphometrically at intervals of 3, 6, and 24 hrs. after harvest. The following features were evaluated using the Toup View software: length, width, surface and perimeter of erythrocytes for species with anucleated erythrocytes. The same characteristics were evaluated in the nucleus for species with nucleated erythrocytes. The data obtained were processed with statistical programs to highlight changes in erythrocyte morphology produced by anticoagulants.