In order to obtain information of the roles of src tyrosine kinase on regenerative mechanisms of neurons after axtomy, we immnuhistochemically investigated changes of p60c-src, phosphtyrosine (pTyr), platelet derived growth factor (PDGF) and PDGF receptor. We also studied western blotting analysis of p60c-srcin the dorsal motor nucleus of vagus, nerve, which shows degeneration after axotomy, and the hypoglossal nucleus, which shows regeneration, after nerve transection in adult rats. 1. Increased immunoreactivity of pTyr, which is the reaction product of tyrosine kinase, is a sensitive marker for detection of the activity of tyrosine kinases, was observed in the axotomized neurons in bothe vagal and hypoglossal nuclei. These results indicate that tyrosine kinases concern with axonal reaction. PDGF decreased in the vagal nucleus after axoltomy, while PDGF receptor, which direct associated with Src protein kinase, showed no remarkable change after axotomy. These findings suggest that decrease of PDGF in vagal neurons result in inactivation of Src protein in neurons after axotomy. Src protein itself ashowed no remarkable changes after axotomy in both nuclei, while immunoreactivity for SH3 region of Src was strongly increased in the hypoglossal nucleus after axotomy, while only slightly in the vagal nucleus. These findings suggest that conformational change of Src, which related with activation Src, occurs after axotomy strongly in reganerative hypoglossal neurons, while only slightly in degenerative vagal neurons. Therefore, the activation of Src is thought to concern with neuronal regeneration after axotomy. 2. Western blotting for Src protein showed no significant changes in both vagal and hypoglossal nuclei after axotomy. This result consistent with the immunohistochemical study.