The social environment can be stressful for at least some group members, resulting in elevated levels of glucocorticoid stress hormones (GC). Patterns of the relationships between social rank and GC levels vary between species. In carnivores, primates and birds that live in permanent cooperative groups, helpers do not usually display physiological indicators of stress. Very little is known about status-related GC differences within cooperative groups of rodents. In this laboratory study, we compared GC concentrations in dominant (fathers) and subordinate (natal sons) males of a cooperative subterranean vole, Ellobius tancrei. The assessment of adrenocortical activity by measuring urine glucocorticoid metabolites (UGM) was previously validated for this species through an ACTH challenge test. We observed clear peaks of UGM in the second or third urine samples taken after the administration of ACTH (lag time equal to 2.5–3 h). Thus, UGM is suitable to estimate physiological stress in Ellobius. Postpubertal sons living in natal groups had significantly higher UGM concentrations than their fathers. The average UGM levels of sons were positively associated with their ages and paternal body masses, and negatively associated with paternal ages. Hence, son-father interactions rather than just younger ages of sons appear to contribute to GC differences. The revealed pattern was not consistent with that reported for most cooperative species from other taxa, highlighting the importance of comparative studies.