We have previously demonstrated that systemic administration of testosterone increases the rate of axonal regeneration following facial nerve crush in adult male hamsters. In the present study, the molecular mechanisms by which androgens could enhance axonal regeneration were examined at a cellular level. Specifically, the following question was addressed using quantitative in situ hybridization with cDNA probes complementary to II, and ␣1 tubulin mRNAs: Does exogenous testosterone augment axotomy-induced changes in tubulin mRNA expression in hamster facial motoneurons (FMN)? Castrated adult male hamsters were subjected to right facial nerve severance, with the left side serving as internal control. One-half of the animals received testosterone replacement in the form of subcutaneously implanted silastic capsules containing crystalline testosterone propionate, and the other half were implanted with blank capsules immediately following the axotomy. Postoperative survival times from 2-14 days were examined. Axotomy alone resulted in a significant increase in the levels of both II and ␣1 tubulin mRNAs in facial motor neurons between 2-14 days after injury. Administration of testosterone selectively augmented the axotomy-induced increases in II-tubulin, but not ␣1 tubulin, mRNA, levels at 7 and 14 days post axotomy. These results demonstrating an effect of testosterone in altering the neuronal cytoskeletal response to axotomy suggest that testosterone may enhance the regenerative properties of motor neurons via molecular mechanisms that involve selective alterations of the neuronal cytoskeleton.