Environmental studies in several countries are including genotoxicity tests as tools to evaluate exposure to contaminants. Most of these tests are developed on sentinel species of mammals. The Ctenomys genus has been used as a sentinel for screening environmental contamination in several countries as Brazil, Chile, Argentina and Uruguay. One of the most used assays has been the in vivo micronucleus test which can be conducted easily on peripheral blood samples since the maturation of erythrocytes involves the loss of the major nucleus. The test potentially can detect genotoxic damage caused by chronic exposures. However, the results obtained may reflect chronic exposure depending on the spleen features that the species have. Until now, there are no studies to determine the role of the spleen in the Ctenomys genus. This note provides new data dealing with this subject and provides an anatomical interpretation of the results obtained. To determine chronic or recently exposure when a sentinel is used, is highly recommended to develop this kind of approaches.