Recent research has described a component of human electrical brain activity (the ERN or NE) that is associated with error-processing. In the present experiment, we used magneto-encephalographic recordings to provide converging evidence both for the existence of this component and for its putative source in the brain. Six human subjects performed a Go-NoGo task while both magnetoencephalographic and electroencephalographic brain activity were recorded. We found evidence for a magnetic equivalent of the ERN and dipole source analysis suggested that this activity was generated in the anterior cingulate cortex. These data converge with those from electrical recordings in implicating this brain structure in error-processing.