Endocrine changes may modulate isolationinduced aggressiveness inMus musculus. Significant differences in the blood levels of T4, T3 and rT3 in males classified as dominant, submissive and reared in isolation have previously been obtained. Isolated mice appear homologous to socially dominant animals as far as aggressiveness is concerned. The endocrinological results support this hypothesis. In that experiment dominant and submissive males, after an open fight, lived together 4 days in a stable hierarchy of two, and were then sacrificed. To find out if levels of peripheral thyroid hormones in dominant and submissive males differ before fighting or depend on living together in a restricted environment of one dominant and one submissive male, the animals in the present experiment were immediately sacrificed after fighting. The results are different from the previous ones, demonstrating that the differences in endocrine functions previously observed are present only when the social hierarchy is stable.