Recent experimental work in animals has emphasized the importance of homeostatic plasticity as a means of stabilising the properties of neuronal circuits. We have established a paradigm that can be used to probe homeostatic-like plasticity at a regional level in the intact human cortex (1,2). The paradigm combines two transcranial stimulation techniques that can produce long-term effects on the excitability of corticospinal output neurones: transcranial direct current stimulation (TDCS) and repetitive transcranial magnetic stimulation (rTMS) of the left primary motor hand area. In healthy individuals, facilitatory pre-conditioning with anodal TDCS caused a subsequent period of rTMS to reduce corticospinal excitability, whereas inhibitory pre-conditioning with cathodal TDCS tuned the conditioning effect of rTMS towards facilitation. Hence, the magnitude and direction of after-effects induced by slow or fast rTMS depended on the state of cortical excitability before stimulation and was tuned by pre-conditioning with TDCS. This implies that changing the initial state of the motor cortex by a period of DC polarisation can reverse the conditioning effects of rTMS. These findings indicate a homeostatic mechanism in human motor cortex that stabilizes corticospinal excitability within a physiologically useful range. This type of metaplasticity is of relevance to the therapeutic application of transcranial cortex stimulation in neuropsychiatric disorders.