The effects of the opioid antagonist naltrexone (NALTX) daily administration (1 mg/kg SC) from birth on the levels of dopamine (DA), serotonin (5-HT), and their respective major metabolites, in the striatum, midbrain, and hypothalamus of 7-, 14-, and 22-day-old rats were investigated. Naltrexone treatment increased the striatal HVA/DA ratio on postnatal day 7. At day 14, two subpopulations (A and B) were found among the treated animals. The subpopulation A showed decreased HVA/DA and increased DOPAC/DA ratios, whereas the subpopulation B presented a higher DA concentration. No significant effect appeared on the striatal dopaminergic system in 22-day-old pups. The serotonergic system was affected by exposure to naltrexone only from day 14. The subpopulation A showed a reduction in all the parameters measured in the three regions studied, although in the subpopulation B, lower 5-HIAA/5-HT ratios appeared in the midbrain and hypothalamus. At 22 days of age NALTX treatment elevated striatal 5-HT and 5-HIAA and the ratio of 5-HIAA/5-HT in the midbrain and hypothalamus. These data suggest an endogenous opioid modulation on the central aminergic systems during the neonatal period and point out the consequences of opioid plasticity on related neurotransmitter systems.