Bilateral deficit, also known as bilateral deficit in movement speed, has been defined as a reduction in force that accompanies maximal two-limb efforts of the homologous limbs relative to single performance; that is, reaction time is longer for simultaneous bilateral responses than for unilateral ones. Although we know that this phenomenon is mediated by neural mechanisms such as interactions between the cerebral hemispheres or spinal reflexes, th e underlying mechanism is still far from clear. The purpose of this study was to examine bilateral deficit in children whose nervous system was not yet completely mature, and to clarify the developmental characteristics of bilateral and unilateral movements.
 Right-handed 11 children aged 6 to 9 participated as subjects and were compared to right-handed the adult subjects aged 18 to 22. The subjects were asked to perform a simple reaction time task by grasping a grip bar in response to an auditory stimulus. The subjects carried out this task uder three conditions: with the right hand(dominant); with left hand(non-dominant); and with both hands simultaneously.
 In the adult subjects, the bilateral RT was consistently shorter than either the left or right unilateral RT. However, the bilateral deficit was not seen in the children to the same extent as in the adults, and, in fact, we found facilitation rather than deficit in some bilateral responses of the children.
 Generally, it is difficult for children to manipulate things separately while they are carrying out a task. In the present study, it was presumed that facilitation rather than inhibition occurred in the children as they carried out a bilateral simultaneous movement, that is, a kind of maximal effort when they responded as fast as possible. We hypothesized that different mechanisms may be responsible for fast bilateral movement in children.