The extent to which the plasticity in peptide expression observed in developing spinal motoneurons occurs following proximal peripheral axotomy in the adult rat was examined using in situ hybridization and immunohistochemical techniques to visualize the changes. Transient upregulation of galanin, vasoactive intestinal polypeptide (VIP) and substance P messenger ribonucleic acids (mRNAs) was observed within subpopulations of motoneurons ipsilateral to lesion for periods lasting 2-3 weeks after injury. In contrast, the axotomy-induced heterogenous increases in somatostatin and neuropeptide tyrosine mRNA expression in ipsilateral motoneurons remained elevated, or, in the case of somatostatin, continued to increase for the time period studied (1 month). Immunohistochemical analysis agreed with the in situ hybridization results, showing some motoneurons within the injured ventral horn to contain galanin-, VIP- or somatostatin-like immunoreactivity. In some instances, galanin-immunoreactive motoneurons colocalized with calcitonin gene-related peptide immunoreactivity. Most of the neurons expressing the injury-induced peptides appeared large, presumably alpha-motoneurons but there were also many small neurons expressing galanin in the ventral horn ipsilateral to lesion. This may represent evidence for peptide synthesis in gamma-motoneurons. The only peptide mRNA studied to be down-regulated in response to axotomy was enkephalin. The results show that peptide expression in injured motoneurons is dramatically altered, the significance of which remains to be determined.