In simple visuomotor reaction time tasks, the difference between reaction time (RT) in the uncrossed hand/hemifield condition from RT in the crossed hand/hemifield, known as CUD (crossed-uncrossed difference), has been interpreted as reflecting interhemispheric transmission time (IHTT). Several studies in normal adults have found a CUD of a few milliseconds (3-4), while an abnormally long CUD has been reported in patients who underwent a surgical section of the corpus callosum or in congenital acallosal subjects. The corpus callosum, which is the most important structure for interhemispheric transfer of information, completes its myelination approximately by age ten. It has been hypothesized that the functional maturity of the corpus callosum coincides with the termination of the myelination cycle. No developmental study has focused on the development of IHTT, in relation to callosal maturation. The purpose of our study has been to investigate the development of interhemispheric transfer of visuomotor information in children aged seven to eleven, using a simple RT task with lateralized visual stimuli. The results indicate an age-related decrease of CUD, which we interpret as reflecting the maturation of the corpus callosum during childhood years.