To evaluate the influence of early malnutrition on the development of behavioral sensitization induced by repeated morphine administration in different groups of control (C-) and deprived (D) rats motor activity was assessed following different doses of morphine (5, 7.5, 10 or 15 mg/kg, i.p.) or saline every other day for 5 days. At every doses, D-rats showed sensitization and a lower number of sessions were needed to induce this phenomenon as compared to C-rats. Moreover, when a challenge of cocaine (10 mg/kg, i.p) was given 48 h after the last morphine administration, only D-rats showed cross-sensitization in morphine-pretreated animals (7.5 and10 mg/kg). When extracellular dopamine (DA) were measured inn. accumbens (core an shell) and in dorsal caudate-putamen following a challenge with cocaine in morphine pre-exposed rats (7,5mg/kg) a higher increase in DA release only in n. accumbens (core) of D-rats was observed. Similar extracellular DA levels were found in the n.accumbens (shell) and in dorsal caudate-putamen of both groups. These results demonstrated that D-rats had a lower thresholdto develop sensitization to morphine and cross-sensitization to cocaine as well as a higher responsiveness of the n. accumbens (core) expressed by increased DA release.