We investigated movement-related cortical potentials from motor cortex areas (C3 and C4) and isometric force and electromyogram (EMG) activity in association with maximal bilateral (BL) and unilateral (UL) handgrip contraction in eight right-handed subjects. The BL grip exhibited deficits in force [right, -5.2 (SEM 1.1)%; left, -4.5 (SEM 1.9)%] and EMG [right, -9.5 (SEM 2.2)%; left, -7.6 (SEM 2.5)%] compared with the UL grip. In the UL contractions, the amplitudes of the negative slope [NS' 2.77 (SEM 0.70) vs 2.40 (SEM 0.76) microV.s for left hand, P < 0.05; 2.54 (SEM 0.55) vs 2.23(SEM 0.54) microV.s for right hand, P < 0.05 and motor potentials [MP: 1.56 (SEM 0.32) microV.s vs 1.23 (SEM 0.35) microV.s for left hand, P < 0.01; 1.44 (SEM 0.32) microV.s vs 1.10 (SEM 0.25) microV.s for right hand, P < 0.01] were greater in the contralateral hemisphere. For the BL contractions, the asymmetry of the larger potentials for the contralateral side disappeared and lower symmetrical potentials [NS', 2.43 (SEM 0.61) microV.s for C3 vs 2.43 (SEM 0.63) microV.s for C4: MP: 1.31 (SEM 0.35) microV.s for C3 vs 1.34 (SEM 0.32) microV.s for C4] were observed. It was concluded that the BL deficit in force and EMG is associated with reduced movement-related cortical potentials suggesting that the bilateral force and (EMG) deficit compared with unilateral handgrip contractions is caused by a mechanism of interhemispheric inhibition.