To elucidate the effects of the decision to move on the pyramidal tract in humans, we examined the changes in the motor evoked potentials (MEP) of the forearm muscles following transcranial magnetic cortical stimulation (TMS) of the hand area during a go/no-go hand-movement task in 10 normal subjects. The subjects performed an extension of the right wrist according to the go, no-go and control signals, one of which was randomly presented on a TV. A single TMS was applied to the primary hand motor area in the left hemisphere 0-300 ms after each signal. The MEPs recorded from the wrist extensor and flexor muscles changed in amplitude after both go and no-go signals. In comparison with the control, the MEPs were significantly facilitated in the agonistic muscles (wrist extensor muscles) and attenuated in the antagonistic muscles (wrist flexor muscles), at the latencies of 100-200 ms after the go signal (P < 0.02). In contrast, the MEPs of both the extensor and flexor muscles were significantly attenuated during the period of 100-200 ms after the no-go signal (P < 0.001). We speculate that there is strong inhibition on the pyramidal tract after the no-go signal and that the inhibitory effect is non-specific to the target muscles. This inhibition differs from the reciprocal inhibition of the MEP observed in antagonistic muscles after the go signal, and it is probably related to the movement decision originating in the prefrontal cortex.