An experimental study was made of the normal force in a magneto rheological suspension under shear flow with the magnetic field applied normally to the shear planes. Under no deformation, the normal force was found to increase with magnetic flux density, and acted to push apart the rheometer plates. At the start-up of shearing under a constant shear rate, the normal force decreased with strain and reached a plateau value which became smaller as shear rate was raised. This behaviour can be interpreted in terms of the aggregate model of magneto rheological suspensions: the normal force arises from the elongated aggregates spanning the gap between the plates, and these are broken up into smaller particle clusters under shearing.