Three alkaloid fractions were obtained from seeds of Erythrina americana: free alkaloids in hexane, free alkaloids in methanol and liberated alkaloids. The pharmacological evaluation of these fractions on rats showed that, administered in a dose of 3 mg/kg, the three fractions diminished the aggressive behavior. This is comparable when diazepam is used as a control. An interaction between the cholinergic and GABAergic system could be suggested.