The excitability of the human motor cortex during the development of implicit and declarative knowledge of a motor task was examined. During a serial reaction time test, subjects developed implicit knowledge of the test sequence, which was reflected by diminishing response times. Motor cortical mapping with transcranial magnetic stimulation revealed that the cortical output maps to the muscles involved in the task became progressively larger until explicit knowledge was achieved, after which they returned to their baseline topography. These results illustrate the rapid functional plasticity of cortical outputs associated with learning and with the transfer of knowledge from an implicit to explicit state.