- This study investigates the timing of bimanual movements in a combined behavioral and physiological approach. Human subjects and rhesus monkeys performed the same bimanual task. In monkeys, we simultaneously recorded neuronal activity in the two hemispheres of primary motor cortex (MI) or supplementary motor area (SMA), and related it to bimanual coordination in the temporal domain. Both for monkeys and humans, the reaction times of bimanual movements never significantly exceeded the reaction times of the slower arm in unimanual movements. Consistent with this, the longest delay between neural activity onset in SMA and MI and movement initiation was observed in unimanual movements of the slower arm and not in bimanual movements. Both results suggest that the programming of bimanual movements does not require more processing time than unimanual movements. They are also consistent with the view that bimanual movements are programmed in a single process, rather than by combining two separate unimanual movement plans. In both humans and monkeys, movement initiation was highly correlated between the arms. However, once movements began, the temporal correlation between the arms progressively declined. Movement decorrelation was accompanied by a net decorrelation of neuronal population activity in MI and SMA, suggesting a functional connection between neuronal interactions and the level of bimanual coupling and decoupling. The similarity of neuronal activities in MI and SMA in relationship to behavioral timing lends support to the idea that both areas are involved in the temporal coordination of the arms.