In the behavioral despair (forced swimming) test and in the tail-suspension test, long-term (30-32 days) castration significantly increased the duration of immobility in mice. Testosterone propionate (1 or 10 mg.kg-1.day sc for 4 days), although not affecting the duration of immobility in sham-operated mice, reduced the duration of immobility in castrated mice to within normal limits. Desipramine (20 mg/kg ip) decreased the duration of immobility both in sham-operated and in castrated animals. These results indicate that castration favors an inactive behavior and that testosterone, although having no "antidepressant" effect per se, is necessary for the male animal to cope normally with adverse environmental situations.