Objective Based on findings in exploration of the rules of plastic changes in MI and SmI of adult rats following total brachial plexus root avulsion injuries, we investigated the central mechanism of transhemispheric reorganization primarily after contralateral seventh cervical nerve root transfer. The role of callus during transhemispheric reorganization progress was also observed.Methods Three adult rat models were set up, including the model of total left brachial plexus root avulsion injury, the nerve transfer model of contralateral seventh cervical nerve root to median nerve, as well as the callosotomy model. Both intracortical microstimulation and somatosensory evoked potential (SEP) were used to study the time course of plastic changes in bilateral primary motor cortex (MI) and somatosensory cortex (SmI) following left total brachial plexus root avulsion injuries, transfer of contralateral seventh cervical nerve root to median nerve and callosotomy at different intervals .Results 1. (1) SEPs were not evoked from bilateral SmI when stimulating the median nerve region of affected forepaw, neither were they evoked from ipsilateral SmI when stimulating the median nerve region of the unaffected forepaw at different intervals following total left brachial plexus root avulsion injuries. On the contrary, SEPs were induced from contralateral SmI of the unaffected forepaw throughout the entire period following total left brachial plexus root avulsion injury. Meanwhile, sites number of the evoked SEPs at different intervals was much more than normal control. (2) Various types of movement were evoked in the former forelimb representation region instead of moment of the disconnected left forelimb. However, only the sites of neck firmly occupied the former forelimb representation region throughout the entire process. These extended representation region obtained the normal function of motor cortex quickly and permanently. The contralateral representation region in MIof the unaffected forelimb extended significantly, and the excitability of the extended region was also enhanced significantly. 2. (1) SEPs were evoked from contralateral SmI of the unaffected forepaw when stimulating the median nerve region of affected forepaw at different intervals after transfer of the contralateral seventh cervical nerve root to median nerve. However, SEPs were not evoked from the ipsilateral SmI when stimulating the unaffected forepaw as well as the contralateral SmI when stimulating the affected forepaw. Interestingly, SEPs were evoked only from the ipsilateral SmI of affected forepaw when stimulating the median nerve region of affected forepaw at 5, 7 and 10 months but 3 months after the nerve transfer. And the evoked SEPs took on an enlargement tendency in amplititude of wave gradually, as well as a reduction tendency in latent period by degree. (2) After stimulating bilateral MI, median nerve representation region of the affected forelimb appeared only in the ipsilateral MI instead of contralateral MI at 5 months after the nerve transfer. At 7 months median nerve representation region of the affected forelimb appeared in MI of both hemispheres. At 10 months after the nerve transfer, however, median nerve representation region of the affected forelimb appeared merely in the contralateral MI, and still lied in the former forelimb representation region. 3. It was not found that the median nerve representation region of affected forelimb to appear in the contralateral MI throughout the entire observation period after callosotomy. However, it appeared merely in the ipsilateral MI at 5, 7 and 10 months after callosotomy. Conclusion 1. The MI and SmI of adult rats still had the ability of plasticity. Dynamic functional reorganization took place in bilateral SmI and MI of adult rats following total brachial plexus root avulsion injury of one forelimb. Special mechanism that maintains the balance and coordination between the corresponding homologous regions of bilateral cerebral cortex existed. The basis for the mechanism might be the exis