This study investigates the control mechanisms at the cortical and spinal levels of antagonist coactivation during a submaximal fatiguing contraction of the elbow flexors at 50 % of maximal voluntary contraction (MVC). We recorded motor evoked potentials in the biceps brachii and triceps brachii muscles in response to magnetic stimulation of the motor cortex (MEP) and corticospinal tract (cervicomedullary motor evoked potentials - CMEP), as well as the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus, before, during and after the fatigue task. The results showed that although the coactivation ratio did not change at task failure, the MVC torque produced by the elbow flexors declined by 48% (P<0.01) with no change in MVC torque for the elbow extensors. While the MEP and CMEP areas (normalized to Mmax) of the biceps brachii increased (~ 50%) over the first 40% of the time to task failure and then plateaued, both responses in the triceps brachii increased (~ 150-180%) gradually throughout the fatigue task. In contrast to the monotonic increase in the MEP and CMEP of the antagonist muscles, the H-reflex of the triceps brachii exhibited a biphasic modulation, increasing during the first part of the contraction before declining subsequently to 65% of its initial value. Collectively, these results suggest that the level of coactivation during a fatiguing contraction is mediated by supraspinal rather than spinal mechanisms and involves differential control of agonist and antagonist muscles.