Oscillatory neuronal synchrony is thought to play an essential role in the neuronal processing of information. Modulation of these ongoing oscillations is a possible mechanism by which processing can selectively be biased. Evidence is presented for a modulation of oscillatory neuronal synchrony by motor set, obtained with magnetoencephalography. Subjects performed a bilateral isometric contraction and were cued in some trials to respond at the event of the imperative go-cue with a left and in others with a right wrist extension. The analysis of the period in which subjects could expect the go-cue to occur (stimulation period) revealed that beta power (15-30 Hz) was lower over the motor cortex when it was contralateral to the expected response side compared to when it was contralateral to the side of which no response was required. This difference did not exist in the baseline period in which the go-cue could not occur. The effect was due to a decrease in beta power in the stimulation period compared to baseline that was bigger for the motor cortex driving the cued side. Force output was equal in the two conditions and stratification of the EMG signals did not change the results. The location of the maximal cortico-muscular coherence in an independent data set was highly similar to the location of the maximal decrease in beta power, and analysis within these channels also showed the same results. We conclude that motor set results in a selective modulation of beta power during a steady bilateral contraction.