Homeostatic mechanisms have been shown to maintain synapses within a dynamic range of modifiability. A plausible explanation for homeostatic plasticity is the Bienenstock-Cooper-Munro (BCM) theory. The BCM model might directly relate to the experimental observation which revealed the influence of the prior neuronal activity on the magnitude and direction of subsequent synaptic plasticity that is called metaplasticity. Here we demonstrate bidirectional long-term modification of MEPs elicited by a burst of four monophasic TMS pulses (quadripulse stimulation: QPS) separated by various inter-pulse intervals of 1.5-1250 ms, and its priming-dependent modulation to reveal metaplasticity of the human primary motor cortex.