Beta-endorphin, Met-enkephalin, substance P, somatostatin and dynorphin concentrations were evaluated in right and left brain areas, and in cervical, thoracic, and lumbosacral spinal cord of rats that underwent section of either the right, the left, both sciatic nerves, the right brachial plexus, the saphenous, or the sural nerve. With all the surgeries, beta-endorphin concentrations decreased significantly in all brain areas with the exception of the striatum where they did not change. By contrast Met-enkephalin increased in all brain areas and in the spinal cord tracts interested by the lesions. The other peptides were always unaffected. The changes in the concentrations of the neuropeptides were observed starting twenty-four hours after surgery and lasted for at least four months. We did not find a lateralization in the brain peptide concentrations of either sham operated or unilaterally deafferentated rats. Moreover, the treatment with serotoninergic agents normalized the concentrations of beta-endorphin, suggesting a role of the serotoninergic system in the decrease of the peptide that follows the lesion of peripheral nerves.