The motor cortex can be stimulated transcranially producing excitatory and inhibitory phenomena in muscles controlled by the activated cortical areas. The physiologic bases of these effects are still relatively poorly understood because of the complexity of the interactions between the currents induced in the brain with an intricate arrangement of neural circuits in the cerebral cortex, which is composed of multiple excitatory and inhibitory networks of cell bodies and axons of different size, location, orientation and function. All forms of stimulation of the intact motor cortex tend to produce repetitive discharge of corticospinal neurones; however, different structures of these central motor circuits seem to be preferentially targeted by the available different techniques of stimulation. Direct recording of the evoked corticospinal output has provided important insight into the excitatory and inhibitory phenomena produced by cerebral cortex stimulation. An updated overview of human and animal studies on the physiologic mechanisms of intact motor cortex stimulation is presented.