Each year, 730,000 Americans experience a stroke. Forty percent are left with persistent impairment of upper extremity function. Although scientifically vetted rehabilitation therapies for this impairment are starting to emerge, current treatment is generally unsatisfactory. Therapies that seek to engage neuroplastic mechanisms constitute one approach to this problem. A good example is constraint induced movement therapy (CIMT), a treatment that seeks, through extensive functional task practice, to overcome an acquired intentional predisposition to use the spared arm (learned non-use), and to improve motor function in the affected arm. CIMT has been tested in a host of trials, most recently a multicenter randomized controlled trial (RCT) - the EXCITE trial. These trials have generally demonstrated that on average, the treatment shows efficacy, and the results from the RCT indicate that it is more efficacious than "standard" therapies. However, problems with CIMT can be readily identified that pose research challenges: 1) on average, efficacy is limited; 2) only a fraction of subjects show substantial benefit. We propose to address these two problems in a pilot RCT of 20 subjects that will test two modifications of standard CIMT: 1) addition of a drug, d-cycloserine, that may enhance neuroplasticity by potentiating NMDA-glutamate receptor-mediated learning mechanisms; 2) delivery of a fixed amount of CIMT over a greater number of days, which according to learning research, may enhance long-term retention of gains.