Lectins isolated from Canavalia brasiliensis seeds (ConBr) and from Canavalia ensiformes seeds, Concanavalin A (ConA) have their crystal protein structure and some biological effects on mammalian cells characterized. This study investigates the action of the central administration of ConBr and ConA in the mouse forced swimming test (FST) , which  is a test widely  used for screening antidepressants. ConBr (1-10 µg/site, i.c.v.,), but not ConA, produced a long-lasting decrease (from 15 min until 2 h) in the immobility time in the FST, without changing the locomotor activity in the open-field test. Heated ConBr (10 mg/site) did not produce a reduction in the immobility time in the FST, indicating that  the effect of ConBr in the FST was dependent on its protein structure integrity. ConBr (0.1 mg/site, i.c.v) caused a potentiation of the action of the classical antidepressant fluoxetine (0.1 nmol/site, i.c.v.), a selective 5-HT reuptake inhibitor. The anti-immobility effect elicited by ConBr (10 mg/site, i.c.v.) in the FST was prevented by the pretreatment of mice with pindolol (32 mg/kg, a 5-HT_1A/1B receptor/b-adrenoceptor antagonist), NAN-190 (0.5 mg/kg, a 5-HT_1A receptor antagonist), ketanserin (5 mg/kg, a 5-HT_2A/2C receptor antagonist), sulpiride (50 mg/kg, a D₂ receptor antagonist), and yohimbine (1 mg/kg, an a₂-adrenoceptor antagonist), but not with SCH 23390 (0.05 mg/kg, a D₁ receptor antagonist), and prazosin (1 mg/kg, an a₁-adrenoceptor antagonist). These results indicate that the antidepressant-like effect of ConBr in the FST is dependent on its interaction with the serotoninergic (via 5-HT_1A and 5-HT₂), noradrenergic (via a₂-adrenoceptors) and dopaminergic (via D₂ receptors) systems. Considering the presence of lectins in the brain and based on the results we might speculate a possible role of endogenous lectins in the modulation of neural function involved in the regulation of mood.