In order to examine the relationship between the action of opioid neurotransmitters and growth factors in the regulation of brain development, we have studied the long-term effect of the opiate antagonist naltrexone (NTX) on the content of nerve growth factor (NGF) in cortex, hippocampus, septum and neostriatum, and on NGF receptor (NGFRs) levels in cortical membranes. 50 mg/kg NTX treatment induced a decrease in the number of 125I-NGF high-affinity binding sites, without detectable changes in NGF levels. However, low doses of NTX (1 mg/kg) produced no differences in 125I-NGF binding sites, but induced a decrease in NGF levels in hippocampus, septum and neostriatum. These results suggest that NGF and NGFRs could be involved in the trophic effects of opioids during brain development.