Interhemispheric inhibition (IHI) is an important mechanism to maximize the independent functioning of each hemisphere and is most likely mediated by transcallosal fibres. IHI can be investigated by paired pulse transcranial magnetic stimulation (TMS) whereby, in half of the trials, a test stimulus (TS) over one hemisphere is preceded by a conditioning stimulus (CS) over the other hemisphere. Whereas various studies have investigated IHI in rest, less is known about interhemispheric interactions during voluntary muscle activation. Here, we investigated the influence of tonic muscle activity (5% of the maximal voluntary contraction) in either the right wrist flexor or extensor versus rest on IHI from the active (left) to the resting (right) hemisphere. Our main finding was that tonic activation of the right wrist flexor, led to an increase in IHI from the active (dominant left) to the resting (non-dominant right) hemisphere as compared to rest. A control experiment employed the same design but CS intensity was lowered to match MEP amplitudes of the conditioning hand between active and rest conditions. This resulted in a relative decrease of IHI. It is hypothesized that functional regulation of IHI might prevent the occurrence of mirror activity in the primary motor cortex (M1) of the resting hemisphere and, thus, might play an important role in the execution of unimanual actions.