OBJECTIVE Cerebral stroke is a major cause for long-term disability in the elderly and it is often manifested in hemiparesis of the upper extremity. Constraint-induced movement therapy (CIMT), an intensive 2 week rehabilitation program, improves affected upper limb motor abilities in subjects with stroke. Intensive training has also been suggested to modify neural function. We wanted to find out if there are changes in cortical excitabihlity in subjects with chronic stroke after CIMT. METHODS Participants were 13 subjects with stroke who fulfilled the entry criteria for the CIMT program. The motor function of the affected arm and hand was assessed using a structured motor behavior test (WMFT). The cortical excitability changes were assessed in regard to involuntary and voluntary motor control, former with transcrania magnetic stimulation (TMS) and latter with movement-related cortical potentials (MRCP). RESULTS The motor abilities of the affected arm improved after 2 week rehabilitation (total time in WMFT shortened 36%). The motor-evoked potential (MEP) amplitudes were unchanged following the stimulation of the non-affected hemisphere; however, the MEP amplitudes of lateral stimulation locations increased significantly in the affected side after the intervention. The power spectra of MRCPs revealed reduced peak frequency over the supplementary motor area when the affected hand was moved. However, no changes occurred when the healthy hand was moved. CONCLUSIONS We show changes in cortical electrical excitability while performing both involuntary and voluntary movements after 2 weeks of CIMT in subjects with chronic stroke. These changes may be seen as a sign of neural reorganization instigated by the intervention.