Summary Chronically stressed adult male Balb C mice were submitted to the tail suspension test. Chronic immobilization stress (6 h/d for 14 consecutive days) induced a significant reduction in immobility time when compared to non-stressed controls. Pretreatment with LY 53857, a serotonin 5HT2 antagonist, and IPS 339, a selective beta-2 adrenoceptor blocker, reversed immobility time to the levels of non-stressed controls. Chronic administration of corticosterone (100 mg/kg for 7 d) did not modify immobility time as compared to saline treated controls. It is suggested that both serotonergic and adrenergic pathways in the brain may participate in the stress-induced changes occurring in the tail suspension test response and that corticosterone does not appear to play a role in this process.