Subjects with partial or complete defects of the corpus callosum, either congenital or acquired, performed a choice reaction time (RT) task involving a right or left key-press response to a light presented at random in the right or left hemifield. Like normal subjects, all of them exhibited two additive effects typical of these tasks: the spatial stimulus-response compatibility effect (faster RT for stimuli and responses matched for side), and the hand placement effect (longer RT for responses performed with crossed hands). Two subjects with a complete callosal defect, one acquired and the other congenital, showed a third effect, not present in normal subjects, consisting of a marked advantage for RT of responses with hand anatomically ipsilateral to the stimulus, independent of both stimulus-response compatibility and hand placement. These findings can be interpreted according to a hierarchical model of information processing assuming that, in the absence of the corpus callosum, the matching of the mental codes for the stimulus and response sets takes place solely in the hemisphere receiving the stimulus, with a subsequent rapid-intrahemispheric or slow-interhemispheric transmission of the response command to the appropriate motor centers.