The effect of unilateral tonic muscle activity with and without coactivation of the antagonists on motor cortex excitability has been studied. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseus muscles of both hands in response to single and paired-pulse transcranial magnetic stimulation (TMS) during relax and during unilateral voluntary tonic activity of the right (dominant) hand (co-activation of antagonists or isometric index finger abduction). The unilateral tonic activation facilitated contralateral and ipsilateral responses (cMEP and iMEP) recorded from both hands with an exception of iMEPs recorded from the right hand. During paired-pulse TMS the intracortical inhibition (ICI) for cMEPs was not influenced by the unilateral tonic activity in both hands, while the intracortical facilitation (ICF) was suppressed when MEPs were recorded from the active right hand. The effect of unilateral tonic activity on iMEP in response to paired-pulse TMS was essentially different: generally, the intracortical inhibition (ICI) was greater for iMEPs and ICF was completely abolished with an exception of iMEPs recorded from the left hand during right finger isometric abduction when a strong ICF was evident. The decreased ICF and/or increased ICI are assumed to reflect mechanisms underlying the co-activation of antagonists.