Malnutrition effect during the suckling period on aggressive behavior was investigated in adult rats treated and not treated with fluoxetine, a selective serotonin reuptake inhibitor. Sixty-four Wistar male rats were allocated in two groups, according to their mothers' diet during lactation. The well-nourished group was fed by mothers receiving a 23% protein diet; the malnourished one by mothers receiving a 8% protein diet. Following weaning, all rats received the 23% protein diet. On the 90th day after birth, each nutritional group was divided into two subgroups, one receiving a single daily injection of fluoxetine (10 mg/kg) and the other of a saline solution (0.9% NaCl) for 14 days. Treatment with Fluoxetine reduced aggressive response in well-nourished but not in malnourished rats. These findings suggest that the serotoninergic system was affected by malnutrition during the critical period of brain development, and persisted even after a long period of nutritional recovery.